Copy & Paste Feature in same Layer but not editable seperately?

First of all, I’m new here and hope this question is accurate enough and isn’t answered already somewhere. Here’s the problem:

I have an existing polygon and need to create another one of it (within the same layer), with a small part of the border edited.
Normaly, I just copy & paste the feature within the same layer. Then I can edit the copied entry and update the attribute table.

But when I do this, somehow QGIS also edits the borders of the original polygon. The entries in the attribute table remain updated, but the polygons are the same again.

The only way this worked was, when I copiy & pasted the polygon into a new temporary scratch layer, edit the borders there and then copied it back into the original layer. But with this method, I have to edit all entries in the attribute table for the copied polygon, altough I only would have to edit one row.

I use QGIS 3.10.8 longterm release.