#314 Post-Python 3: Duplicate Delete Recognized Domain submissions can be approved

[Ahasuerus]

If a bureaucrat creates a duplicate Delete Recognized Domain submission, i.e. two submissions that would delete the same Recognized Domain, they can be approved one after the other. This bug is fairly harmless since the second approval would simply delete a no-longer existing Recognized Domain. However, the filer code should still be updated to check that the submitted Recognized Domain exists and, if it doesn't, make the submission unapprovable.