diff --git a/llvm/lib/Transforms/IPO/FunctionImport.cpp b/llvm/lib/Transforms/IPO/FunctionImport.cpp
--- a/llvm/lib/Transforms/IPO/FunctionImport.cpp
+++ b/llvm/lib/Transforms/IPO/FunctionImport.cpp
@@ -282,10 +282,25 @@
     FunctionImporter::ImportMapTy &ImportList,
     StringMap<FunctionImporter::ExportSetTy> *ExportLists) {
   for (auto &VI : Summary.refs()) {
-    if (DefinedGVSummaries.count(VI.getGUID())) {
-      LLVM_DEBUG(
-          dbgs() << "Ref ignored! Target already in destination module.\n");
-      continue;
+    // Find if the module already contains a suitable definition.
+    const auto &GVS = DefinedGVSummaries.find(VI.getGUID());
+    if (GVS != DefinedGVSummaries.end()) {
+      if (VI.getSummaryList().size() > 1 &&
+          GlobalValue::isInterposableLinkage(GVS->second->linkage())) {
+        // If the module contains a non-prevailing def of a multiply defined
+        // global, it still requires a prevailing one to be imported. Otherwise,
+        // it may result in an undefined symbol because if the prevailing copy
+        // is read-only it will be internalized thus become not available for
+        // linking.
+        // FIXME: Instead of checking for linkage type, use prevailing symbol
+        // information and ensure the prevailing copy is read-only before allow
+        // importing.
+        ;
+      } else {
+        LLVM_DEBUG(
+            dbgs() << "Ref ignored! Target already in destination module.\n");
+        continue;
+      }
     }
 
     LLVM_DEBUG(dbgs() << " ref -> " << VI << "\n");
diff --git a/llvm/test/ThinLTO/X86/Inputs/weak_globals_import.ll b/llvm/test/ThinLTO/X86/Inputs/weak_globals_import.ll
new file mode 100644
--- /dev/null
+++ b/llvm/test/ThinLTO/X86/Inputs/weak_globals_import.ll
@@ -0,0 +1,11 @@
+target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+@G = dso_local local_unnamed_addr global i32 42, align 4
+
+; Function Attrs: noinline
+define dso_local i32 @_Z3foov() local_unnamed_addr #0 {
+  ret i32 0
+}
+
+attributes #0 = { noinline }
diff --git a/llvm/test/ThinLTO/X86/weak_globals_import.ll b/llvm/test/ThinLTO/X86/weak_globals_import.ll
new file mode 100644
--- /dev/null
+++ b/llvm/test/ThinLTO/X86/weak_globals_import.ll
@@ -0,0 +1,30 @@
+; RUN: opt -module-summary %s -o %t1.bc
+; RUN: opt -module-summary %p/Inputs/weak_globals_import.ll -o %t2.bc
+
+; RUN: llvm-lto2 run -save-temps %t1.bc %t2.bc -o %t.out \
+; RUN:               -r=%t1.bc,main,plx \
+; RUN:               -r=%t1.bc,_Z3foov,l \
+; RUN:               -r=%t1.bc,G \
+; RUN:               -r=%t2.bc,_Z3foov,pl \
+; RUN:               -r=%t2.bc,G,pl
+; RUN: llvm-dis %t.out.1.3.import.bc -o -  | FileCheck %s
+; RUN: llvm-dis %t.out.2.3.import.bc -o -  | FileCheck %s
+
+; Test that a prevailing def for a global variable got imported even if
+; a destination module already contains non-prevailing one.
+
+; CHECK: @G = internal local_unnamed_addr global i32 42
+
+target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+@G = weak dso_local local_unnamed_addr global i32 0, align 4
+
+define dso_local i32 @main() local_unnamed_addr {
+  %1 = load i32, i32* @G, align 4
+  %2 = tail call i32 @_Z3foov()
+  %3 = add nsw i32 %2, %1
+  ret i32 %3
+}
+
+declare dso_local i32 @_Z3foov() local_unnamed_addr