If I go to
https://en.wikibooks.org/wiki/Special:Book

create a collection, add some pages and then try to save it under the same name of an existing collection I get a warning about that:

A page with the name Wikibooks:Collections/Acoustics already exists. Do you want it to be replaced with your collection?

but there should be a diff somewhere, so I can decide what to do.

Version: unspecified
Severity: enhancement