go/analysis: validate: report duplicates among analyzers (roots)
go/analysis: validate: report duplicates among analyzers (roots)

Duplicate root analyzers caused duplicate flag registration and other problems.

Change-Id: Id0c2761529c57ed1f9a63b669e62401ebf035cc2
Reviewed-on: https://go-review.googlesource.com/c/141159
Reviewed-by: Michael Matloob <4562438b12c14e81a47550acde616ff328a909f8@golang.org>

golang/tools,golang/tools,golang/tools,golang/tools,golang/tools