Treatment of duplicate integrity constraints

[tohrnii]

Currently we don't throw an error if a user defines duplicate constraints, we transpile them into two equivalent constraints. We could do one of the following:

1. Generate only one constraint in this case and ignore duplicates.
2. Throw an error since this is most likely unintended.
3. Throw a warning (in future) but still generate one constraint.

[bobbinth]

In my opinion, throwing an error is probably a better option.