Index: lld/ELF/Driver.h
===================================================================
--- lld/ELF/Driver.h
+++ lld/ELF/Driver.h
@@ -46,6 +46,8 @@
   std::unique_ptr<BitcodeCompiler> lto;
 
   std::vector<InputFile *> files;
+
+  llvm::StringSet<> addedLibraries;
 };
 
 // Parses command line options.
Index: lld/ELF/Driver.cpp
===================================================================
--- lld/ELF/Driver.cpp
+++ lld/ELF/Driver.cpp
@@ -201,6 +201,13 @@
 void LinkerDriver::addFile(StringRef path, bool withLOption) {
   using namespace sys::fs;
 
+  // If we have already added the file as a library, no need to do that again.
+  // However, there are a few cases when the file might be processed in
+  // a different way and the optimization should be avoided.
+  if (!config->warnBackrefs && !config->formatBinary && !inWholeArchive &&
+      addedLibraries.contains(path))
+    return;
+
   Optional<MemoryBufferRef> buffer = readFile(path);
   if (!buffer.hasValue())
     return;
@@ -216,6 +223,11 @@
     readLinkerScript(mbref);
     return;
   case file_magic::archive: {
+    // The optimization is incompatible with --warn-backrefs; do not waste time
+    // filling the set in that case.
+    if (!config->warnBackrefs)
+      addedLibraries.insert(path);
+
     // Handle -whole-archive.
     if (inWholeArchive) {
       for (const auto &p : getArchiveMembers(mbref))
Index: lld/test/ELF/print-archive-stats.s
===================================================================
--- lld/test/ELF/print-archive-stats.s
+++ lld/test/ELF/print-archive-stats.s
@@ -20,7 +20,8 @@
 # RUN: ld.lld %t.o %tweak.a %t1.a --print-archive-stats=- -o /dev/null | diff %t.txt -
 
 ## The second %t1.a has 0 fetched member.
-# RUN: ld.lld %t.o %tweak.a %t1.a %t1.a --print-archive-stats=- -o /dev/null | \
+## Note: --warn-backrefs disables the optimization which skips repeated archives.
+# RUN: ld.lld %t.o %tweak.a %t1.a %t1.a --print-archive-stats=- --warn-backrefs -o /dev/null | \
 # RUN:   FileCheck --check-prefix=CHECK2 %s
 # CHECK2:      members	fetched	archive
 # CHECK2-NEXT: 1	0	{{.*}}weak.a
Index: lld/test/ELF/skip-repeated-libs.s
===================================================================
--- /dev/null
+++ lld/test/ELF/skip-repeated-libs.s
@@ -0,0 +1,26 @@
+# REQUIRES: x86
+
+# RUN: llvm-mc -filetype=obj -triple=x86_64 %s -o %t.o
+# RUN: echo ".global foo; foo:" | \
+# RUN:   llvm-mc -filetype=obj -triple=x86_64 - -o %t1.o
+# RUN: rm -f %t1.a && llvm-ar rc %t1.a %t1.o
+
+## The linker can resolve backrefs, so the repeated adding of the library can be skipped.
+# RUN: ld.lld %t1.a %t1.a %t.o -o /dev/null --verbose 2>&1 | FileCheck %s
+# CHECK:     {{.*}}1.a
+# CHECK-NOT: {{.*}}1.a
+
+## If the repeated definition is given after -whole-archive, it should not be skipped.
+# RUN: ld.lld %t1.a -whole-archive %t1.a %t.o -o /dev/null --verbose 2>&1 | FileCheck %s --check-prefix=WHA
+# WHA:      {{.*}}1.a
+# WHA-NEXT: {{.*}}1.a
+
+## If, for any reason, the second definition is given after -format binary,
+## it should not be skipped.
+# RUN: ld.lld %t.o %t1.a -format binary %t1.a -o %t
+# RUN: llvm-nm %t | FileCheck --check-prefix=BINARY %s
+# BINARY: D _binary_{{.+}}1_a_start
+
+  .global _start
+_start:
+  call foo