I made some test with Krita 5.2.9 on Windows 10 / 11.
I have not seen the “gradient duplication” but there is an issue with respect to vector vs. bitmap layers.
I hope the screenshots explain it:
EDIT:
I have confirmed the bug report, even though my findings are not exactly the same like yours @SimonH
But I think, they have the same root cause.
EDIT2:
The gradient preview is also affected by the opacity values:
