At  the moment according to the knowledge base If you are importing lists from multiple sources,  Net Community will not check for duplicates. (KB 47252) Condition A states ` Condition A: A message utilizes lists from multiple data sources.
Result A: Duplicate emails may be sent as there is no check made against each other.’

I think it would be a good idea if the data was checked for duplicates as this would improve the experience for donors as well as cutting down the admin time required  to manually De-dupe the forms