So, to fix the random numbers you generated (I used the RAND function in my example below), select the cells containing those ...

If your source data is in Excel and has duplicates, run Power Query to remove those duplicates before running a Word mail merge.

You can remove duplicates in Excel in a few steps. Duplicates can create problems when you're dealing with data.

Remove duplicate entries from Microsoft Excel worksheets with add-in Duplicate Remover for Excel.