"same name" = duplicated map labels.

The implication is that a map label describes a location in the world, and
therefore must be world-unique. This is an assumption made by the engine which
was recently broken by Matthias, by mistake. We have a level validator just for
this kind of things so it's sad that it doesn't cover the unicity of map labels.
Given that we've just been bitten by the issue, I think now is a good time to
add unicity checks to the level validator.