Hmm, actually the test case is even simpler as plain assembly, just:

    .comm   FOO,1,1
    .comm   FOO,1,1

will already result in the assertion:

$ ~/obj/llvm-237905-trunk-freebsd11-i386-ninja-rel-1/bin/clang -cc1as -triple x86_64-unknown-freebsd -filetype obj comm-twice.s
Assertion failed: (!isCommon()), function getOffset, file /share/dim/src/llvm/trunk/include/llvm/MC/MCSymbol.h, line 69.
Abort trap

While this construct is obviously silly, I don't think clang should assert on it.

Note that GNU as assembles this just fine, of course:

$ as -V comm-twice.s -o comm-twice.o
GNU assembler version 2.25 (x86_64-portbld-freebsd11.0) using BFD version (GNU Binutils) 2.25

$ objdump -x comm-twice.o

comm-twice.o: file format elf64-x86-64-freebsd
comm-twice.o
architecture: i386:x86-64, flags 0x00000010:
HAS_SYMS
start address 0x0000000000000000

Sections:
Idx Name Size VMA LMA File off Algn
0 .text 00000000 0000000000000000 0000000000000000 00000040 20
CONTENTS, ALLOC, LOAD, READONLY, CODE
1 .data 00000000 0000000000000000 0000000000000000 00000040 20
CONTENTS, ALLOC, LOAD, DATA
2 .bss 00000000 0000000000000000 0000000000000000 00000040 2**0
ALLOC
SYMBOL TABLE:
0000000000000000 l d .text 0000000000000000 .text
0000000000000000 l d .data 0000000000000000 .data
0000000000000000 l d .bss 0000000000000000 .bss
0000000000000001 O COM 0000000000000001 FOO

Fixed in 239227