spec: state that variable names must be unique in short var decls
spec: state that variable names must be unique in short var decls

Fixes #45652.

Change-Id: I5e1434480c12815369a6ce204f3729eb63139125
Reviewed-on: https://go-review.googlesource.com/c/go/+/405757
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@google.com>
Reviewed-by: Matthew Dempsky <e56ac2c500064424afac424d4b5e53bd0570a731@google.com>

nawawi/go,nawawi/go,golang/go,nawawi/go,golang/go,nawawi/go,golang/go,nawawi/go,golang/go,golang/go,nawawi/go,golang/go,golang/go,golang/go,nawawi/go,nawawi/go