There is a simple way if you can use code and the target table is set to not allow duplicates. You simply run the append and rather than get the standard message that n records can't be processed, you suppress the warning. Whatever is new gets appended. Whatever is not new is bypassed. The warning can be suppressed by turning them on and off if using RunSql method (not my favourite thing to do) or CurrentDb.Execute without using dbFailOnError option.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.