As a Debian package maintainer or other interested party, on a package
page I can see a warning when an NMU is the most recent version of the
given package. (Ticket migrated from Trello:
https://trello.com/c/duNhR7pO/)

Acceptance Tests:

  If it is detected that the version of the source package matches a
  source NM upload, a todo item is added.

  It asks that the non maintainer upload is incorporated and acknowledged.

  Should this include all repositories or only the default?

Only the default. And it's debatable whether this is really relevant
nowadays.

A better version of this would:
1/ check if the package uses a VCS repository, and if yes complain only if
   the NMU is not committed to the VCS repository
2/ give this message only for packages where no VCS repository is used

Hence this could be integrated with the recent vcswatch.

Cheers,