silencing warning about duplicate attributes

[catamorphism]

In syntax::attr::require_unique_names, @pcwalton asked in a comment "How do I silence the warnings?" Actually, the warning is an error, so I'm not exactly sure what was meant...

[alexcrichton]

@pcwalton Do you remember the reason for this warning?

It seems like it may be outdated and this could be closed?

[bstrie]

pcwalton says this can be closed.