[2/6] widl: Ignore assigment of a duplicate uuid.

[Huw Davies]

On Thu, Oct 15, 2015 at 08:46:46PM +0800, Dmitry Timoshkov wrote:
> MSDN KB285146 article describes this behaviour, and the tests in the last
> patch confirm that.

That article also mentions that the warning doesn't occur when the udt is
an enum (and indeed midl doesn't issue a warning in this case).  We should
do likewise.

[Also, while you're at it, please fix the typo in subject: 'assigment']

Huw.