Import numbers and alphanumeric

I use windows and filemaker 5.5

I have two fields and both are text fields. I need to import data from excel into these fields but I have to use import update matching records in current found set and add new records because I may have records in my excel sheet that have to be updated and some new data too. In field 1 I have only numbers and in field 2 I have numbers and letters. When I use field 2 as the matching field it imports the matching records again instead of just updating them, in other word, it duplicated the records that exist in both excel and FileMaker. When I use field 1 as the matching field it imports correctly, updating the matching field without duplicating the field in Filemaker. I've tested doing the import using both fields but the one I need to import and use as the matching field is the one that duplicates the records and its the one that contains the alphanumeric data.Please, why does this happen and how can I solve this problem ?

It's not about the current found records because I've done show all before importing in both ways.