Add UniqueList annotated type

[JimMadge]

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⤴️ Summary

🌂 Related issues

Closes #1721

🔬 Tests

[jemrobinson]

LGTM. One question about how the type definition works.

[jemrobinson]

Given that a UniqueList is actually a set() could/should we be using set everywhere instead?

[JimMadge]

Given that a UniqueList is actually a set() could/should we be using set everywhere instead?

That is a good question. I think we don't want to do that here because we are parsing user input. If they provide an invalid input, we should raise an error rather than trying to fix it. set would remove duplicates rather than raising an exception.