Attribution model data should be unique to prevent duplicates

[bpepple]

Describe the bug
Right now there is no validation done on new Attribution objects to verify the data isn't duplicated.

To Reproduce
Noticed this when I update the 'Marvel Update' command, and got duplicate Attribution data.

Expected behavior
New Attribution objects shouldn't be created.

Additional context
Need to either add a unique_together constraint or other type of check.

[bpepple]

For now, dd96825 should fix the 'update_marvel' command, but should look further into whether it makes sense to add a constraint of some kind to the model.