Index: lld/test/wasm/duplicate-function-imports.s
===================================================================
--- /dev/null
+++ lld/test/wasm/duplicate-function-imports.s
@@ -0,0 +1,57 @@
+# RUN: llvm-mc -filetype=obj -triple=wasm32-unknown-unknown %s -o %t.o
+# RUN: wasm-ld -o %t1.wasm %t.o
+# RUN: obj2yaml %t1.wasm | FileCheck %s
+
+.globl f1
+.import_module f1, env
+.import_name f1, f
+.functype f1 () -> (i32)
+
+# Same import module/name/type as `f1`, should be de-duped.
+.globl f2
+.import_module f2, env
+.import_name f2, f
+.functype f2 () -> (i32)
+
+# Same import module/name, but different type. Should not be de-duped.
+.globl f3
+.import_module f3, env
+.import_name f3, f
+.functype f3 () -> (f32)
+
+.globl _start
+_start:
+  .functype _start () -> ()
+  call f1
+  drop
+  call f2
+  drop
+  call f3
+  drop
+  end_function
+
+
+# CHECK:        - Type:            TYPE
+# CHECK-NEXT:     Signatures:
+# CHECK-NEXT:       - Index:           0
+# CHECK-NEXT:         ParamTypes:      []
+# CHECK-NEXT:         ReturnTypes:
+# CHECK-NEXT:           - I32
+# CHECK-NEXT:       - Index:           1
+# CHECK-NEXT:         ParamTypes:      []
+# CHECK-NEXT:         ReturnTypes:
+# CHECK-NEXT:           - F32
+# CHECK-NEXT:       - Index:           2
+# CHECK-NEXT:         ParamTypes:      []
+# CHECK-NEXT:         ReturnTypes:     []
+# CHECK-NEXT:   - Type:            IMPORT
+# CHECK-NEXT:     Imports:
+# CHECK-NEXT:       - Module:          env
+# CHECK-NEXT:         Field:           f
+# CHECK-NEXT:         Kind:            FUNCTION
+# CHECK-NEXT:         SigIndex:        0
+# CHECK-NEXT:       - Module:          env
+# CHECK-NEXT:         Field:           f
+# CHECK-NEXT:         Kind:            FUNCTION
+# CHECK-NEXT:         SigIndex:        1
+# CHECK-NEXT:   - Type:
Index: lld/test/wasm/duplicate-global-imports.s
===================================================================
--- /dev/null
+++ lld/test/wasm/duplicate-global-imports.s
@@ -0,0 +1,69 @@
+# RUN: llvm-mc -filetype=obj -triple=wasm32-unknown-unknown %s -o %t.o
+# RUN: wasm-ld --no-check-features -o %t1.wasm %t.o
+# RUN: obj2yaml %t1.wasm | FileCheck %s
+
+.global g1
+.import_module g1, env
+.import_name g1, g
+.globaltype g1, i64, immutable
+
+# Same import module/name/type as `g1`, should be de-duped.
+.global g2
+.import_module g2, env
+.import_name g2, g
+.globaltype g2, i64, immutable
+
+# Imported as an i32 instead of i64, so should not be de-duped.
+.global g3
+.import_module g3, env
+.import_name g3, g
+.globaltype g3, i32, immutable
+
+# Imported as mutable instead of immutable, so should not be de-duped.
+.global g4
+.import_module g4, env
+.import_name g4, g
+.globaltype g4, i64
+
+.globl _start
+_start:
+  .functype _start () -> ()
+  global.get g1
+  drop
+  global.get g2
+  drop
+  global.get g3
+  drop
+  global.get g4
+  drop
+  end_function
+
+
+# CHECK:        - Type:            IMPORT
+# CHECK-NEXT:     Imports:
+# CHECK-NEXT:      - Module:          env
+# CHECK-NEXT:        Field:           g
+# CHECK-NEXT:        Kind:            GLOBAL
+# CHECK-NEXT:        GlobalType:      I64
+# CHECK-NEXT:        GlobalMutable:   false
+# CHECK-NEXT:      - Module:          env
+# CHECK-NEXT:        Field:           g
+# CHECK-NEXT:        Kind:            GLOBAL
+# CHECK-NEXT:        GlobalType:      I32
+# CHECK-NEXT:        GlobalMutable:   false
+# CHECK-NEXT:      - Module:          env
+# CHECK-NEXT:        Field:           g
+# CHECK-NEXT:        Kind:            GLOBAL
+# CHECK-NEXT:        GlobalType:      I64
+# CHECK-NEXT:        GlobalMutable:   true
+# CHECK-NEXT:  - Type:
+
+# CHECK:         GlobalNames:
+# CHECK-NEXT:      - Index:           0
+# CHECK-NEXT:        Name:            g1
+# CHECK-NEXT:      - Index:           1
+# CHECK-NEXT:        Name:            g3
+# CHECK-NEXT:      - Index:           2
+# CHECK-NEXT:        Name:            g4
+# CHECK-NEXT:      - Index:           3
+# CHECK-NEXT:        Name:            __stack_pointer
Index: lld/test/wasm/duplicate-table-imports.s
===================================================================
--- /dev/null
+++ lld/test/wasm/duplicate-table-imports.s
@@ -0,0 +1,94 @@
+# RUN: llvm-mc -mattr=+reference-types -triple=wasm32-unknown-unknown -filetype=obj -o %t.o %s
+# RUN: wasm-ld --no-entry --export-all --allow-undefined -o %t1.wasm %t.o
+# RUN: obj2yaml %t1.wasm | FileCheck %s
+
+.globl t1
+.import_module t1, env
+.import_name t1, t
+.tabletype t1, funcref
+
+# Same import module/name/type as `t1`, should be de-duped.
+.globl t2
+.import_module t2, env
+.import_name t2, t
+.tabletype t2, funcref
+
+# Imported as an externref instead of funcref, so should not be de-duped.
+.globl t3
+.import_module t3, env
+.import_name t3, t
+.tabletype t3, externref
+
+.globl read_funcref_1
+read_funcref_1:
+  .functype read_funcref_1 () -> (funcref)
+  i32.const 0
+  table.get t1
+  end_function
+
+.globl read_funcref_2
+read_funcref_2:
+  .functype read_funcref_2 () -> (funcref)
+  i32.const 0
+  table.get t2
+  end_function
+
+.globl read_externref
+read_externref:
+  .functype read_externref () -> (externref)
+  i32.const 0
+  table.get t3
+  end_function
+
+# XXX: the second imported table has index 1, not 0. I've verified by hand (with
+# `wasm2wat`) that the resulting Wasm file is correct: `t3` does end up at index
+# 1 and our `read_*` functions contain `table.get` instructions using the proper
+# table index immediates. This is also asserted (less legibly) in the hexdump of
+# their code bodies below. It looks like there's a bug in how `obj2yaml`
+# disassembles multiple table imports.
+
+# CHECK:        - Type:            IMPORT
+# CHECK-NEXT:     Imports:
+# CHECK-NEXT:       - Module:          env
+# CHECK-NEXT:         Field:           t
+# CHECK-NEXT:         Kind:            TABLE
+# CHECK-NEXT:         Table:
+# CHECK-NEXT:           Index:           0
+# CHECK-NEXT:           ElemType:        FUNCREF
+# CHECK-NEXT:           Limits:
+# CHECK-NEXT:             Minimum:         0x0
+# CHECK-NEXT:       - Module:          env
+# CHECK-NEXT:         Field:           t
+# CHECK-NEXT:         Kind:            TABLE
+# CHECK-NEXT:         Table:
+# CHECK-NEXT:           Index:           0
+# CHECK-NEXT:           ElemType:        EXTERNREF
+# CHECK-NEXT:           Limits:
+# CHECK-NEXT:             Minimum:         0x0
+# CHECK-NEXT:   - Type:
+
+# CHECK:        - Type:            CODE
+# CHECK-NEXT:     Functions:
+# CHECK-NEXT:       - Index:           0
+# CHECK-NEXT:         Locals:          []
+# CHECK-NEXT:         Body:            0B
+# CHECK-NEXT:       - Index:           1
+# CHECK-NEXT:         Locals:          []
+# CHECK-NEXT:         Body:            41002580808080000B
+# CHECK-NEXT:       - Index:           2
+# CHECK-NEXT:         Locals:          []
+# CHECK-NEXT:         Body:            41002580808080000B
+# CHECK-NEXT:       - Index:           3
+# CHECK-NEXT:         Locals:          []
+# CHECK-NEXT:         Body:            41002581808080000B
+# CHECK-NEXT:   - Type:            CUSTOM
+# CHECK-NEXT:     Name:            name
+# CHECK-NEXT:     FunctionNames:
+# CHECK-NEXT:       - Index:           0
+# CHECK-NEXT:         Name:            __wasm_call_ctors
+# CHECK-NEXT:       - Index:           1
+# CHECK-NEXT:         Name:            read_funcref_1
+# CHECK-NEXT:       - Index:           2
+# CHECK-NEXT:         Name:            read_funcref_2
+# CHECK-NEXT:       - Index:           3
+# CHECK-NEXT:         Name:            read_externref
Index: lld/wasm/SyntheticSections.h
===================================================================
--- lld/wasm/SyntheticSections.h
+++ lld/wasm/SyntheticSections.h
@@ -96,6 +96,68 @@
   llvm::DenseMap<WasmSignature, int32_t> typeIndices;
 };
 
+/**
+ * A key for some kind of imported entity of type `T`.
+ *
+ * Used when de-duplicating imports.
+ */
+template <typename T> struct ImportKey {
+public:
+  enum class State { Plain, Empty, Tombstone };
+
+public:
+  T type;
+  llvm::Optional<StringRef> importModule;
+  llvm::Optional<StringRef> importName;
+  State state;
+
+public:
+  ImportKey(T type) : type(type), state(State::Plain) {}
+  ImportKey(T type, State state) : type(type), state(state) {}
+  ImportKey(T type, llvm::Optional<StringRef> importModule,
+            llvm::Optional<StringRef> importName)
+      : type(type), importModule(importModule), importName(importName),
+        state(State::Plain) {}
+};
+
+template <typename T>
+inline bool operator==(const ImportKey<T> &lhs, const ImportKey<T> &rhs) {
+  return lhs.state == rhs.state && lhs.importModule == rhs.importModule &&
+         lhs.importName == rhs.importName && lhs.type == rhs.type;
+}
+
+} // namespace wasm
+} // namespace lld
+
+// `ImportKey<T>` can be used as a key in a `DenseMap` if `T` can be used as a
+// key in a `DenseMap`.
+template <typename T> struct llvm::DenseMapInfo<lld::wasm::ImportKey<T>> {
+  static lld::wasm::ImportKey<T> getEmptyKey() {
+    typename lld::wasm::ImportKey<T> key(llvm::DenseMapInfo<T>::getEmptyKey());
+    key.state = lld::wasm::ImportKey<T>::State::Empty;
+    return key;
+  }
+  static lld::wasm::ImportKey<T> getTombstoneKey() {
+    typename lld::wasm::ImportKey<T> key(llvm::DenseMapInfo<T>::getEmptyKey());
+    key.state = lld::wasm::ImportKey<T>::State::Tombstone;
+    return key;
+  }
+  static unsigned getHashValue(const lld::wasm::ImportKey<T> &key) {
+    uintptr_t hash = hash_value(key.importModule);
+    hash = hash_combine(hash, key.importName);
+    hash = hash_combine(hash, llvm::DenseMapInfo<T>::getHashValue(key.type));
+    hash = hash_combine(hash, key.state);
+    return hash;
+  }
+  static bool isEqual(const lld::wasm::ImportKey<T> &lhs,
+                      const lld::wasm::ImportKey<T> &rhs) {
+    return lhs == rhs;
+  }
+};
+
+namespace lld {
+namespace wasm {
+
 class ImportSection : public SyntheticSection {
 public:
   ImportSection() : SyntheticSection(llvm::wasm::WASM_SEC_IMPORT) {}
@@ -131,6 +193,9 @@
   unsigned numImportedFunctions = 0;
   unsigned numImportedTags = 0;
   unsigned numImportedTables = 0;
+  llvm::DenseMap<ImportKey<WasmGlobalType>, uint32_t> importedGlobals;
+  llvm::DenseMap<ImportKey<WasmSignature>, uint32_t> importedFunctions;
+  llvm::DenseMap<ImportKey<WasmTableType>, uint32_t> importedTables;
 };
 
 class FunctionSection : public SyntheticSection {
Index: lld/wasm/SyntheticSections.cpp
===================================================================
--- lld/wasm/SyntheticSections.cpp
+++ lld/wasm/SyntheticSections.cpp
@@ -107,17 +107,59 @@
   gotSymbols.push_back(sym);
 }
 
+template <typename UndefinedT>
+static void getImportModuleAndName(Symbol *sym, StringRef *outModule,
+                                   StringRef *outName) {
+  if (auto *undef = dyn_cast<UndefinedT>(sym)) {
+    *outModule = undef->importModule ? *undef->importModule : defaultModule;
+    *outName = undef->importName ? *undef->importName : sym->getName();
+  } else {
+    *outModule = defaultModule;
+    *outName = sym->getName();
+  }
+}
+
 void ImportSection::addImport(Symbol *sym) {
   assert(!isSealed);
-  importedSymbols.emplace_back(sym);
-  if (auto *f = dyn_cast<FunctionSymbol>(sym))
-    f->setFunctionIndex(numImportedFunctions++);
-  else if (auto *g = dyn_cast<GlobalSymbol>(sym))
-    g->setGlobalIndex(numImportedGlobals++);
-  else if (auto *t = dyn_cast<TagSymbol>(sym))
+  StringRef module;
+  StringRef name;
+  if (auto *f = dyn_cast<FunctionSymbol>(sym)) {
+    getImportModuleAndName<UndefinedFunction>(sym, &module, &name);
+    ImportKey<WasmSignature> key(*(f->getSignature()), module, name);
+    auto entry = importedFunctions.try_emplace(key, numImportedFunctions);
+    if (entry.second) {
+      importedSymbols.emplace_back(sym);
+      f->setFunctionIndex(numImportedFunctions++);
+    } else {
+      f->setFunctionIndex(entry.first->second);
+    }
+  } else if (auto *g = dyn_cast<GlobalSymbol>(sym)) {
+    getImportModuleAndName<UndefinedGlobal>(sym, &module, &name);
+    ImportKey<WasmGlobalType> key(*(g->getGlobalType()), module, name);
+    auto entry = importedGlobals.try_emplace(key, numImportedGlobals);
+    if (entry.second) {
+      importedSymbols.emplace_back(sym);
+      g->setGlobalIndex(numImportedGlobals++);
+    } else {
+      g->setGlobalIndex(entry.first->second);
+    }
+  } else if (auto *t = dyn_cast<TagSymbol>(sym)) {
+    // NB: There's currently only one possible kind of tag, and no
+    // `UndefinedTag`, so we don't bother de-duplicating tag imports.
+    importedSymbols.emplace_back(sym);
     t->setTagIndex(numImportedTags++);
-  else
-    cast<TableSymbol>(sym)->setTableNumber(numImportedTables++);
+  } else {
+    auto *table = cast<TableSymbol>(sym);
+    getImportModuleAndName<UndefinedTable>(sym, &module, &name);
+    ImportKey<WasmTableType> key(*(table->getTableType()), module, name);
+    auto entry = importedTables.try_emplace(key, numImportedTables);
+    if (entry.second) {
+      importedSymbols.emplace_back(sym);
+      table->setTableNumber(numImportedTables++);
+    } else {
+      table->setTableNumber(entry.first->second);
+    }
+  }
 }
 
 void ImportSection::writeBody() {
Index: llvm/include/llvm/BinaryFormat/Wasm.h
===================================================================
--- llvm/include/llvm/BinaryFormat/Wasm.h
+++ llvm/include/llvm/BinaryFormat/Wasm.h
@@ -67,11 +67,20 @@
   uint64_t Maximum;
 };
 
+inline bool operator==(const WasmLimits &LHS, const WasmLimits &RHS) {
+  return LHS.Flags == RHS.Flags && LHS.Minimum == RHS.Minimum &&
+         LHS.Maximum == RHS.Maximum;
+}
+
 struct WasmTableType {
   uint8_t ElemType;
   WasmLimits Limits;
 };
 
+inline bool operator==(const WasmTableType &LHS, const WasmTableType &RHS) {
+  return LHS.ElemType == RHS.ElemType && LHS.Limits == RHS.Limits;
+}
+
 struct WasmTable {
   uint32_t Index;
   WasmTableType Type;
Index: llvm/include/llvm/BinaryFormat/WasmTraits.h
===================================================================
--- llvm/include/llvm/BinaryFormat/WasmTraits.h
+++ llvm/include/llvm/BinaryFormat/WasmTraits.h
@@ -63,6 +63,47 @@
   }
 };
 
+// Traits for using WasmLimits in a DenseMap
+template <> struct DenseMapInfo<wasm::WasmLimits> {
+  static wasm::WasmLimits getEmptyKey() {
+    return wasm::WasmLimits{0xff, 0xff, 0xff};
+  }
+  static wasm::WasmLimits getTombstoneKey() {
+    return wasm::WasmLimits{0xee, 0xee, 0xee};
+  }
+  static unsigned getHashValue(const wasm::WasmLimits &Limits) {
+    unsigned Hash = hash_value(Limits.Flags);
+    Hash = hash_combine(Hash, Limits.Minimum);
+    Hash = hash_combine(Hash, Limits.Maximum);
+    return Hash;
+  }
+  static bool isEqual(const wasm::WasmLimits &LHS,
+                      const wasm::WasmLimits &RHS) {
+    return LHS == RHS;
+  }
+};
+
+// Traits for using WasmTableType in a DenseMap
+template <> struct DenseMapInfo<wasm::WasmTableType> {
+  static wasm::WasmTableType getEmptyKey() {
+    return wasm::WasmTableType{0,
+                               DenseMapInfo<wasm::WasmLimits>::getEmptyKey()};
+  }
+  static wasm::WasmTableType getTombstoneKey() {
+    return wasm::WasmTableType{
+        1, DenseMapInfo<wasm::WasmLimits>::getTombstoneKey()};
+  }
+  static unsigned getHashValue(const wasm::WasmTableType &TableType) {
+    return hash_combine(
+        TableType.ElemType,
+        DenseMapInfo<wasm::WasmLimits>::getHashValue(TableType.Limits));
+  }
+  static bool isEqual(const wasm::WasmTableType &LHS,
+                      const wasm::WasmTableType &RHS) {
+    return LHS == RHS;
+  }
+};
+
 } // end namespace llvm
 
 #endif // LLVM_BINARYFORMAT_WASMTRAITS_H