Not sure why I'm getting the CSV import error, "This record contains duplicated key or keys"

Hello,

[UPDATE: forgot to mention, I'm using the "overwrite sublist" setting on the import]

Each week I run a CSV import to updated existing purchase order transactions. Roughly 9 transactions are updated by the CSV import, but one most weeks one (and only one) transaction fails, giving the error "This record contains duplicated key or keys. Please correct it before next update."

I'm not using an external id for any of these transactions, and I'm using the internal id to match them in the CSV. I'm not aware of any other keys to check. Any ideas?

When this happens, I end up manually deleting the transaction that failed to update in NS, and running the CSV import under ADD (rather than update). That works, but it's extra steps I was trying to cut out of my weekly tasks.