If I have to use the same criteria and operator more than once in a query then it automatically returns zero results even though I know we have multiple records that match the criteria I've put in.

Blackbaud have told me "Unfortunately the query often has difficulty when using the same criteria more than once, especially when using the AND option which is why we have the Merge Queries function to combat this. "

Although I was already using merge queries as a way round it would be far easier if RE allowed you to use the same criteria multiple times.