Fix #6117 handle duplicate contact types

[thejonroberts]

What github issue is this PR for, if any?

Resolves #6117

What changed, and why?

See Issue and/or bugsnag

Duplicate contact type ids are being shown in the form, due to a join... And if they are sent in request, it will fail because of a unique validation.

This PR:

• adds logic in CaseContactParams class to remove duplicate contact type ids in the request. This is the "main" fix.
• adds .distinct calls when contact types are queried for the form. This should prevent the form showing repeated contact types, and prevent the 'bad' request from happening. Not necessary to stop the error, but a better user experience.
• adds request spec for duplicate type ids in request params. The setup gave the same error seen in bugsnag before the fixes above were applied.

How is this tested? (please write tests!) 💖💪

Third bullet point above. I didn't figure out how the production data resulted in duplication in the form, so I did not make a 'frontend' spec.