I suggest importing into a different table that is linked to the target table into which you are currently attempting to import data by the fields you are selecting as "match" fields in the database.
You can then process data from this table to update matching records and to identify records that do not match without having them present in the target table.
You can either import the data twice, once as import matching into the target table and once into this additional table, or you can import into the additional table and then import from it into your target table after cleaning up and fixing the discrepancies found when you check to see which records match and which ones don't.
But I don't see how you will idenify records that fail to match due to data entry errors from correctly entered data that does not yet occur in your target table. It seems to me that they could easily be indistinguishable from each other.
Thanks for the suggestion,
I decided to go with a duplicate database and import into those with a Match current recorda and add remaining records with one field blank. Then call a find of those that were imported but are missing data in that blank field. Therefore allowing me to check these records for incorrect data entry. Way better than looking at thousands of records for side by side comparison!
Thanks for the suggestion :)