cmd/link: symbol listed multiple times

[bradfitz]

Weird trybot failure at https://storage.googleapis.com/go-build-log/246e316c/windows-386-2008_23d43c6f.log on unrelated https://go-review.googlesource.com/c/go/+/139258

##### Testing packages.
ok  	archive/tar	0.133s
ok  	archive/zip	1.667s
ok  	bufio	0.230s
ok  	bytes	2.659s
ok  	compress/bzip2	0.169s
ok  	compress/flate	0.910s
ok  	compress/gzip	0.405s
ok  	compress/lzw	0.145s
ok  	compress/zlib	1.091s
ok  	container/heap	0.061s
ok  	container/list	0.080s
ok  	container/ring	0.117s
ok  	context	1.069s
ok  	crypto	0.058s
ok  	crypto/aes	0.077s
ok  	crypto/cipher	0.039s
ok  	crypto/des	0.049s
ok  	crypto/dsa	0.066s
ok  	crypto/ecdsa	0.339s
ok  	crypto/elliptic	0.142s
ok  	crypto/hmac	0.081s
ok  	crypto/internal/subtle	0.048s
ok  	crypto/md5	0.051s
ok  	crypto/rand	0.088s
ok  	crypto/rc4	0.105s
ok  	crypto/rsa	0.202s
ok  	crypto/sha1	0.039s
ok  	crypto/sha256	0.035s
ok  	crypto/sha512	0.062s
ok  	crypto/subtle	0.049s
# crypto/tls_test [crypto/tls.test]
duplicate go.string..gostring.586.8c81bd317af8a03b8ef0851ccb074eb17d1ad589b540446244a5e593f78ef820
<autogenerated>:1: symbol go.string..gostring.586.8c81bd317af8a03b8ef0851ccb074eb17d1ad589b540446244a5e593f78ef820 listed multiple times
FAIL	crypto/tls [build failed]
ok  	crypto/x509	2.213s
ok  	database/sql	0.620s
ok  	database/sql/driver	0.034s
2018/10/11 18:26:44 Failed: exit status 2

[alexbrainman]

Another windows-amd64-2012 failure

https://build.golang.org/log/a013ea34c38bccdec508314fb7420b15716bfd1c

ok  	mime	0.060s
ok  	mime/multipart	0.349s
ok  	mime/quotedprintable	0.137s
ok  	net	11.266s
# net/http_test [net/http.test]
duplicate go.string."Hello world!\n"
<autogenerated>:1: symbol go.string."Hello world!\n" listed multiple times
FAIL	net/http [build failed]
2018/10/12 23:23:21 Failed: exit status 2

Alex

[gopherbot]

Change https://golang.org/cl/141648 mentions this issue: cmd/compile: emit symbol for constant string before parallel compiler phase