Add tests to prevent duplicated schema entries.

[jh-RLI]

Thank you for finding all the redundancies and little errors! Maybe it's possible to write a test for duplicates in future.

Originally posted by @christian-rli in #117 (comment)

[jh-RLI]

@christian-rli can you tell me a bit more about the test you suggested. As far as I understand, the test would look for duplicate entries in schmea.json?

[christian-rli]

There isn't more to tell. You just described the suggested test in its entirety :)

[jh-RLI]

Ah okay, just any duplocate (like key names). I thought you had an example as I didn't follow the discussion before :)

[christian-rli]

I looked into the commits again. the naming suggested duplicate ("redundant") fields. I seems like there weren't actually any, but it's a useful test to be implemented anyway (and an easy one, in case someone needs to learn how to write tests).