Warn user when applying a duplication

[barrh]

When rebasing and applying patches, it is possible that git would fail to identify a patch is already included within the target upstream release. Resulting in full/partial code duplication - without conflict.
This could be avoided by comparing downstream-only patch names and/or their (pre-rebase) content, with patches that were added in upstream since the last rebase.

[yazug]

if you have a -patches branch git does this work for you.