Fundamental to the UCC is a provision in most states that you can pay items in any order, which suggests you can, on a given day, decide which items will be returned if you run into one of these multi-channel presentment problems. My tendancy would be to bounce the RDC item if you are able to determine which one it is. It is more likely to be the right selection of "bag holder" if there are innocent third parties involved (such as the check casher in your initial scenario). It goes back to the the account of the probably fraudster.
This is the type of situation the Fed illustrated in its most recent revision to its proposed Reg CC amendments (it's anyone's guess when we're going to see a final rule). One of the options in the proposal is to create an indemnity scheme protecting the bank accepting the paper item for deposit when, through mistake or fraud, the same item has previously been deposited via RDC one or more times. If the bank taking the paper item for deposit gets it back unpaid (because it was previously paid via the RDC deposit channel), the paper-accepting depositary bank would have an indemnity claim against any bank that took the item via RDC, got credit for it and didn't get it back unpaid.
The Fed's theory in proposing the indemnity is that the bank deploying RDC services introduced the risk of error or fraud resulting in multiple presentments, and that bank (or those banks) should build the added risk into their business model for RDC. Put more simply, it would be a case of "You made this bed, now you can lie in it."
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John S. Burnett
BankersOnline.com
Fighting for Compliance since 1976
Bankers' Threads User #8