If I understand the definitions correctly, Point-of-Sale transactions and Point-of-Purchase transactions are not the same. So, if a POS transaction has been identified as a duplicate by a customer, is it returned as a R10 or am I completely confusing the return process for disputed POS items?

Although the POS and POP items come from different origins (POP has a physical check as its source), you can use the R10 return reason code for both, as long as the debit in question falls within the 60-day timeframe. See page OG 74 of the 2003 NACHA rules book.

You may also be able to use R24 "Duplicate Entry". You just need to be sure that the Originator has not already discovered the error and done a reversal before you do your return.