Index: lld/trunk/COFF/InputFiles.cpp
===================================================================
--- lld/trunk/COFF/InputFiles.cpp
+++ lld/trunk/COFF/InputFiles.cpp
@@ -501,6 +501,10 @@
   ExternalName = ExtName;
 
   ImpSym = Symtab->addImportData(ImpName, this);
+  // If this was a duplicate, we logged an error but may continue;
+  // in this case, ImpSym is nullptr.
+  if (!ImpSym)
+    return;
 
   if (Hdr->getType() == llvm::COFF::IMPORT_CONST)
     static_cast<void>(Symtab->addImportData(Name, this));
Index: lld/trunk/test/COFF/duplicate-imp-func.s
===================================================================
--- lld/trunk/test/COFF/duplicate-imp-func.s
+++ lld/trunk/test/COFF/duplicate-imp-func.s
@@ -0,0 +1,43 @@
+# REQUIRES: x86
+
+# RUN: echo -e ".globl libfunc\n.text\nlibfunc:\nret" > %t.lib.s
+# RUN: llvm-mc -triple=x86_64-windows-gnu %t.lib.s -filetype=obj -o %t.lib.o
+# RUN: lld-link -lldmingw -dll -out:%t.lib.dll -entry:libfunc %t.lib.o -implib:%t.lib.dll.a
+
+# RUN: echo -e ".globl helper1\n.text\nhelper1:\ncall libfunc\nret" > %t.helper1.s
+# RUN: echo -e ".globl helper2\n.text\nhelper2:\nret\n.globl libfunc\n.globl __imp_libfunc\nlibfunc:\nret\n.data\n__imp_libfunc:\n.quad libfunc" > %t.helper2.s
+# RUN: llvm-mc -triple=x86_64-windows-gnu %t.helper1.s -filetype=obj -o %t.helper1.o
+# RUN: llvm-mc -triple=x86_64-windows-gnu %t.helper2.s -filetype=obj -o %t.helper2.o
+
+# RUN: llvm-ar rcs %t.helper.a %t.helper1.o %t.helper2.o
+
+# RUN: llvm-mc -triple=x86_64-windows-gnu %s -filetype=obj -o %t.main.o
+
+# Simulate a setup, where two libraries provide the same import function;
+# %t.lib.dll.a is a pure import library which provides "libfunc".
+# %t.helper.a is a static library which contains "helper1" and "helper2".
+#
+# helper1 contains an undefined reference to libfunc. helper2 contains a
+# fake local implementation of libfunc, together with the __imp_libfunc
+# stub.
+#
+# %t.lib.dll.a is listed before %t.helper.a on the command line. After
+# including helper1, the library member that first declared the Lazy libfunc
+# (%t.lib.dll.a) gets enqueued to be loaded. Before that gets done, helper2
+# gets loaded, which also turns out to provide a definition of libfunc.
+# Once the import library member from %t.lib.dll.a gets loaded, libfunc
+# and __imp_libfunc already are defined.
+
+# Just check that this fails cleanly (doesn't crash).
+# RUN: not lld-link -lldmingw -out:%t.main.exe -entry:main %t.main.o %t.lib.dll.a %t.helper.a
+
+# Test with %t.helper.a on the command line; in this case we won't try to
+# include libfunc from %t.lib.dll.a and everything works fine.
+# RUN: lld-link -lldmingw -out:%t.main.exe -entry:main %t.main.o %t.helper.a %t.lib.dll.a
+
+    .globl main
+    .text
+main:
+    call helper1
+    call helper2
+    ret