cmd/vet: check for duplicate json, xml struct field tags

cmd/vet: check for duplicate json, xml struct field tags

It is easy to make the mistake of duplicating json struct field
tags especially when copy/pasting. This commit causes go vet to
report the mistake. Only field tags in the same struct type are
considered, because that is the only case which is undoubtedly an
error.

Fixes #12791.

Change-Id: I4130e4c04b177694cc0daf8f1acaf0751d4f062b
Reviewed-on: https://go-review.googlesource.com/16704
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>