This may not really answer your questions, and furthermore,
it may not feasible for your true datasource and row count,
but wanted to offer the approach of joining your datasource to itself.
This will allow for one-on-one comparisons between workbooks
and calculations on the amount of overlap between them.
Please see the workbook attached in the Forum thread.
Thanks Swaroop for looking at the problem. The solution you provided was a good attempt but not quite what I'm looking for.