Releasy never complains about pre-existing packages

[bil-bas]

If you generate a new version of a build/package, (whether the existing one is broken or built correctly), then Releasy generally gets in a mess and either does nothing or corrupts the existing build. It should complain about the file already existing and, perhaps, offer to delete it for you before proceeding.

Reported by jwoertink (well, he reminded me of the existing problem that I was avoiding dealing with :D).