Since you wish to add the current selection to an existing extract file, you will be first asked to identify that extract file:

The selected extract file currently contains 73 records, by adding the current selection this could increase to 76 if there are no duplicates.

After clicking on the Select button, and reselecting the Add to existing extract file you will see there are now 76 records in this extract file.

If you try to add these 3 records again to this extract file you will find no increase in the number pf records since no duplicates are allowed in extract files.