Enhancement request : detect repeated #:def with the same signature

[bhourahine]

If there is an inadvertent copy-paste mistake in the code to be pre-processed, it would be useful to get a warning/error for repeated definitions of the same macro, either within the same file or the same scope.

A follow up, would be a new keyword to redefine an existing macro (again with warning/error., if the macro has not been previously defined).