When I drag and drop an image from my file browser, or from most webpages (ex, Wikipedia), everything works fine: one copy of the image gets pasted.
However, when I drag and drop an image from some websites, two copies of the image get pasted, and I have to manually press Backspace to delete the duplicate.
Random example that I picked that demonstrates this bug (dragging the pic of the bus into a field in Anki): 경기광주 버스 9000 - 나무위키
This looks like a problem in the website’s HTML; It has some several weird img tags nested in span elements.
EDIT: Also can’t reproduce the problem like @zhushenli. Maybe it depends on your Anki version or add-ons. Though I believe the root cause is the website.