To transfer clients for the first time from 2019 to 2020....open 2020 Program - select from the top left : File then Transfer from the dropdown menu or select Transfer icon from second row
The only reason to have an overwrite issue is if you have already transferred 2019 to 2020 and need to re-transfer. If that is the case, I would delete the 2020 files, then transfer.
Hope this helps
There used to be a checkbox somewhere to allow overwriting, not sure if Ive seen it in 2020 though.