* (bug 20765) Special:ListGroupRights no longer misses addables and removables groups...

* (bug 20765) Special:ListGroupRights no longer misses addables and removables groups if there are duplicate entries
I could reproduce the problem locally, but since I don't know the exact issue on the Polish Wikipedia, I suspect this is this one:
array_unique() doesn't change keys, so if you have:
array(
 0 => 'One',
 1 => 'One',
 2 => 'Two'
)
you'll get after array_unique():
array(
 0 => 'One',
 2 => 'Two'
)
which confuses Language::listToText() since it expects consecutive keys