The reports I get from my bank need manual editing of details after import (payee and other information is mixed in there, resulting in hundreds of new payees at every import...). However, as soon as they are changed, they are added again to the list after the next import process (the csv list from my bank includes all transactions that were ever made). Of course the problem lies with the csv of the bank, but maybe something can be done about it?
With csv inmport, I don't believe there is any way for KMM to identify the new and previous transactions as the same, because it does that using the bank's identifier, which is only used for QIF or OFX imports. If your bank can produce either of those it would be much better. Given your current process, I suspect that you are changing some of the information that KMM would use to match the reimported transaction to the previously imported one. Specifically, if you have changed the name of the payee, how can it know it is the same transaction? Is there no way to filter the export from the bank to a limited time range? If not, then one possible short term workaround would be to open the csv in LibreOffice Calc (or equivalent) and sort by date and then delete the older transactions. Just be sure to save back as csv and not in the application's native spreadsheet format.
The CSV importer has undergone massive changes since the 4.8.0 release. Not sure, if such a feature exists. As with other import channels that do not provide a unique ID from the bank (e.g. KBanking) the importer can construct such a unique ID itself and keep it with the transaction. Even if you change the memo field it can detect that. In case you need to change the name, make sure the one provided by the bank is contained in the list of matching payees. I don't know if this has been incorporated into the new CSV importer.
This works in master branch. This won't be available in 4.8 branch, so marking unmaintained. What I did was: 1) import bank statement, 2) accept imported transactions, 3) change payee names in KMM, 4) import bank statement from 1), Result: No transactions added, no payees added, so KMM identified bank statement as already imported.