Validate that a binding mode is not defined more than once. (#2633)

Validate that a binding mode is not defined more than once. (#2633)

While defining the same mode usually wouldn't hurt and, in fact, the old behavior
allows to split the definition of a binding mode into several blocks, this
can lead to user errors where they accidentally define a mode twice and don't understand
why the mode behaves a certain way (this has been observed in real life :-)).

There's no good usecase for splitting a single binding mode into multiple blocks, thus
the new behavior is better.

fixes #2615