[Xtend] No check for duplicate interface specification

[LorenzoBettini]

While working on #2349 I discovered that Xtend does not check for duplicate interface specification:

This leads to a wrong Java class generation:

And then the Java error is reported on the Xtend file

But maybe we could catch it before getting to the code generation.

@szarnekow I can fix it upstream and then port it to #2349 , what do you think?

[szarnekow]

@LorenzoBettini Yes, please add a validation.