Hi @Jzitek,
This is a good idea - I’m often importing over the top of existing data and having to manually correct.
I’d recommend you post this in the ideas board - https://community.hubspot.com/t5/HubSpot-Ideas/idb-p/HubSpot_Ideas
as it’s more likely to be picked up by developers there.
Ouf of interest, what data is it that’s being overwritten? I have some manual work-arounds I use to sanitise my data pre import which might help.