This is following #11492
This code duplication goes back to 4 years ago, in 783deae.
Back then, ActiveSupport::Concern had a #use method, which was removed later on in the profit of the currently used native #include.
This use method was a bit intrusive.
Therefore, it quite made sense to test that reincluding a module when it has already been didn't just remove it.
Since we now rely on the native include method now, though, there is no use for this anymore.
remove useless duplication in include test