Index: ELF/SyntheticSections.h
===================================================================
--- ELF/SyntheticSections.h
+++ ELF/SyntheticSections.h
@@ -27,6 +27,8 @@
 #include "llvm/ADT/MapVector.h"
 #include "llvm/MC/StringTableBuilder.h"
 
+#include <set>
+
 namespace lld {
 namespace elf {
 
@@ -511,7 +513,7 @@
   GdbHashTab SymbolTable;
 
   // The CU vector portion of the constant pool.
-  std::vector<std::vector<std::pair<uint32_t, uint8_t>>> CuVectors;
+  std::vector<std::set<uint32_t>> CuVectors;
 
   std::vector<AddressEntry> AddressArea;
 
Index: ELF/SyntheticSections.cpp
===================================================================
--- ELF/SyntheticSections.cpp
+++ ELF/SyntheticSections.cpp
@@ -1788,11 +1788,10 @@
     std::tie(IsNew, Sym) = SymbolTable.add(Hash, Offset);
     if (IsNew) {
       Sym->CuVectorIndex = CuVectors.size();
-      CuVectors.push_back({{CuId, Pair.second}});
-      continue;
+      CuVectors.push_back({});
     }
 
-    CuVectors[Sym->CuVectorIndex].push_back({CuId, Pair.second});
+    CuVectors[Sym->CuVectorIndex].insert(CuId | (Pair.second << 24));
   }
 }
 
@@ -1816,7 +1815,7 @@
   ConstantPoolOffset =
       SymTabOffset + SymbolTable.getCapacity() * SymTabEntrySize;
 
-  for (std::vector<std::pair<uint32_t, uint8_t>> &CuVec : CuVectors) {
+  for (std::set<uint32_t> &CuVec : CuVectors) {
     CuVectorsOffset.push_back(CuVectorsSize);
     CuVectorsSize += OffsetTypeSize * (CuVec.size() + 1);
   }
@@ -1869,14 +1868,11 @@
   }
 
   // Write the CU vectors into the constant pool.
-  for (std::vector<std::pair<uint32_t, uint8_t>> &CuVec : CuVectors) {
+  for (std::set<uint32_t> &CuVec : CuVectors) {
     write32le(Buf, CuVec.size());
     Buf += 4;
-    for (std::pair<uint32_t, uint8_t> &P : CuVec) {
-      uint32_t Index = P.first;
-      uint8_t Flags = P.second;
-      Index |= Flags << 24;
-      write32le(Buf, Index);
+    for (uint32_t Val : CuVec) {
+      write32le(Buf, Val);
       Buf += 4;
     }
   }
Index: test/ELF/gdb-index-dup-types.s
===================================================================
--- test/ELF/gdb-index-dup-types.s
+++ test/ELF/gdb-index-dup-types.s
@@ -0,0 +1,60 @@
+# REQUIRES: x86
+# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t.o
+# RUN: ld.lld --gdb-index %t.o -o %t
+# RUN: llvm-dwarfdump -debug-dump=gdb_index %t | FileCheck %s
+
+## Testcase is based on output produced by gcc version 5.4.1 20160904
+## it has duplicate entries in .debug_gnu_pubtypes which seems to be
+## compiler bug. In that case it is useless to have them in .gdb_index
+## and we filter such entries out to reduce size of .gdb_index.
+
+## CHECK: Constant pool offset = {{.*}}, has 1 CU vectors:
+## CHECK-NOT: 0(0x0): 0x90000000 0x90000000
+
+.section .debug_abbrev,"",@progbits
+ .byte 1                       # Abbreviation Code
+ .byte 17                      # DW_TAG_compile_unit
+ .byte 0                       # DW_CHILDREN_no
+ .byte 16                      # DW_AT_stmt_list
+ .byte 23                      # DW_FORM_sec_offset
+ .ascii "\260B"                # DW_AT_GNU_dwo_name
+ .byte 14                      # DW_FORM_strp
+ .byte 27                      # DW_AT_comp_dir
+ .byte 14                      # DW_FORM_strp
+ .ascii "\264B"                # DW_AT_GNU_pubnames
+ .byte 25                      # DW_FORM_flag_present
+ .ascii "\261B"                # DW_AT_GNU_dwo_id
+ .byte 7                       # DW_FORM_data8
+ .ascii "\263B"                # DW_AT_GNU_addr_base
+ .byte 23                      # DW_FORM_sec_offset
+ .byte 0                       # EOM(1)
+ .byte 0                       # EOM(2)
+ .byte 0                       # EOM(3)
+
+.section .debug_info,"",@progbits
+.Lcu_begin0:
+ .long 32                       # Length of Unit
+ .short 4                       # DWARF version number
+ .long .debug_abbrev            # Offset Into Abbrev. Section
+ .byte 8                        # Address Size (in bytes)
+ .byte 1                        # Abbrev [1] 0xb:0x19 DW_TAG_compile_unit
+ .long 0                        # DW_AT_stmt_list
+ .long 0                        # DW_AT_GNU_dwo_name
+ .long 0                        # DW_AT_comp_dir
+ .quad 0                        # DW_AT_GNU_dwo_id
+ .long 0                        # DW_AT_GNU_addr_base
+
+.section .debug_gnu_pubtypes,"",@progbits
+.long .LpubTypes_end0-.LpubTypes_begin0 # Length of Public Types Info
+.LpubTypes_begin0:
+ .short 2                      # DWARF Version
+ .long .Lcu_begin0             # Offset of Compilation Unit Info
+ .long 36                      # Compilation Unit Length
+ .long 36                      # DIE offset
+ .byte 144                     # Kind: TYPE, STATIC
+ .asciz "int"                  # External Name
+ .long 36                      # DIE offset
+ .byte 144                     # Kind: TYPE, STATIC
+ .asciz "int"                  # External Name
+ .long 0                       # End Mark
+.LpubTypes_end0: