I'm trying to update the item details of Credit Memos via CSV import. I'm using internal IDs but some records are failing. The error says "This record contains duplicated key or keys. Please correct it before next update." Can't find anything useful on SAs.

Have you tried line IDs?

No. Because I have overwrite sublist on. These CMs mostly have duplicate line items, so I'm trying to wipe them out and replace with the correct one, so I didn't use line ID and turn on overwrite sublist.