Index: COFF/Config.h
===================================================================
--- COFF/Config.h
+++ COFF/Config.h
@@ -94,7 +94,8 @@
   bool DoICF = true;
   bool TailMerge;
   bool Relocatable = true;
-  bool Force = false;
+  bool ForceUnresolved = false;
+  bool ForceMultiple = false;
   bool Debug = false;
   bool DebugDwarf = false;
   bool DebugGHashes = false;
Index: COFF/Driver.cpp
===================================================================
--- COFF/Driver.cpp
+++ COFF/Driver.cpp
@@ -944,9 +944,11 @@
     Config->Verbose = true;
   errorHandler().Verbose = Config->Verbose;
 
-  // Handle /force or /force:unresolved
+  // Handle /force, /force:unresolved, and /force:multiple
   if (Args.hasArg(OPT_force, OPT_force_unresolved))
-    Config->Force = true;
+    Config->ForceUnresolved = true;
+  if (Args.hasArg(OPT_force, OPT_force_multiple))
+    Config->ForceMultiple = true;
 
   // Handle /debug
   if (Args.hasArg(OPT_debug, OPT_debug_dwarf, OPT_debug_ghash)) {
Index: COFF/InputFiles.cpp
===================================================================
--- COFF/InputFiles.cpp
+++ COFF/InputFiles.cpp
@@ -55,7 +55,8 @@
   if (auto *U = dyn_cast<Undefined>(Source)) {
     if (U->WeakAlias && U->WeakAlias != Target)
       Symtab->reportDuplicate(Source, F);
-    U->WeakAlias = Target;
+    else
+      U->WeakAlias = Target;
   }
 }
 
@@ -430,7 +431,7 @@
   // If type is function, we need to create a thunk which jump to an
   // address pointed by the __imp_ symbol. (This allows you to call
   // DLL functions just like regular non-DLL functions.)
-  if (Hdr->getType() == llvm::COFF::IMPORT_CODE)
+  if (Hdr->getType() == llvm::COFF::IMPORT_CODE && ImpSym)
     ThunkSym = Symtab->addImportThunk(Name, ImpSym, Hdr->Machine);
 }
 
Index: COFF/Options.td
===================================================================
--- COFF/Options.td
+++ COFF/Options.td
@@ -97,8 +97,9 @@
 def wholearchive_flag : F<"wholearchive">;
 
 def force : F<"force">,
-    HelpText<"Allow undefined symbols when creating executables">;
+    HelpText<"Allow undefined or duplicate symbols when creating executables">;
 def force_unresolved : F<"force:unresolved">;
+def force_multiple : F<"force:multiple">;
 defm WX : B<"WX", "Treat warnings as errors", "Don't treat warnings as errors">;
 
 defm allowbind : B<"allowbind", "Enable DLL binding (default)",
Index: COFF/SymbolTable.cpp
===================================================================
--- COFF/SymbolTable.cpp
+++ COFF/SymbolTable.cpp
@@ -59,8 +59,8 @@
   Driver->parseDirectives(S);
 }
 
-static void errorOrWarn(const Twine &S) {
-  if (Config->Force)
+static void errorOrWarn(bool just_warn, const Twine &S) {
+  if (just_warn)
     warn(S);
   else
     error(S);
@@ -171,7 +171,7 @@
 
     // Remaining undefined symbols are not fatal if /force is specified.
     // They are replaced with dummy defined symbols.
-    if (Config->Force)
+    if (Config->ForceUnresolved)
       replaceSymbol<DefinedAbsolute>(Sym, Name, 0);
     Undefs.insert(Sym);
   }
@@ -181,7 +181,8 @@
 
   for (Symbol *B : Config->GCRoot) {
     if (Undefs.count(B))
-      errorOrWarn("<root>: undefined symbol: " + B->getName());
+      errorOrWarn(Config->ForceUnresolved,
+                  "<root>: undefined symbol: " + B->getName());
     if (Config->WarnLocallyDefinedImported)
       if (Symbol *Imp = LocalImports.lookup(B))
         warn("<root>: locally defined symbol imported: " + Imp->getName() +
@@ -195,7 +196,8 @@
       if (!Sym)
         continue;
       if (Undefs.count(Sym))
-        errorOrWarn("undefined symbol: " + Sym->getName() +
+        errorOrWarn(Config->ForceUnresolved,
+                    "undefined symbol: " + Sym->getName() +
                     getSymbolLocations(File, SymIndex));
       if (Config->WarnLocallyDefinedImported)
         if (Symbol *Imp = LocalImports.lookup(Sym))
@@ -253,8 +255,9 @@
 }
 
 void SymbolTable::reportDuplicate(Symbol *Existing, InputFile *NewFile) {
-  error("duplicate symbol: " + toString(*Existing) + " in " +
-        toString(Existing->getFile()) + " and in " + toString(NewFile));
+  errorOrWarn(Config->ForceMultiple,
+              "duplicate symbol: " + toString(*Existing) + " in " +
+              toString(Existing->getFile()) + " and in " + toString(NewFile));
 }
 
 Symbol *SymbolTable::addAbsolute(StringRef N, COFFSymbolRef Sym) {
@@ -353,7 +356,7 @@
   }
 
   reportDuplicate(S, F);
-  return nullptr;
+  return dyn_cast<DefinedImportData>(S);
 }
 
 DefinedImportThunk *SymbolTable::addImportThunk(StringRef Name,
@@ -369,7 +372,7 @@
   }
 
   reportDuplicate(S, ID->File);
-  return nullptr;
+  return dyn_cast<DefinedImportThunk>(S);
 }
 
 std::vector<Chunk *> SymbolTable::getChunks() {