Unique ID attribute not merging lists

Hey Guys,

I have created a primary list with query (Status =A,C,CS,P),(PropertyType=LND,MUL,RES)
and Unique ID attribute as the primary key in the MLS ‘Matrix_Unique_ID’.
ie get everything in the database that is available and residential

I have then exported the import file and re-imported it (duplicating the query and mapping).

I have then changed the imported query to
(MatrixModifiedDT=[yesterday]+),(PropertyType=LND,MUL,RES)

so we have 2 lists, 1 that purges every week, and 1 that runs daily to get inflight changes.

So from my understanding, the Unique ID attribute should be then deduping against the full existing database. we have ~4000 records that have been created in the primary list, and then 300 or so which should be updating on a daily basis with the secondary.

Our issue is that each import list appears to be deduping against its own list rather than the broader database.

Second question.
Assuming we can resolve the above
As we combine multiple MLS databases, we run the risk of having duplicate MLS ids over multiple MLS sources for the different regions. The MLS Number and the keys are unique to each MLS provider, but there is a risk of duplicates in the eyes of the rules above.
We have the MLS ID / Unique key which is the id for each MLS source, I am hoping we can dedup on a concat of MLSID-MLSNumber, thus isolating each MLS source ie TOR-123546798.