I've seen old threads with Alt + M solution, in documentation armature -> merge, but in 2.9 both ways doesn't work. I need to - obviously - delete second shin and 2nd thigh bone. Yup, rigify.
I managed to fix my skeleton by using Vertex Weight Mix modifier. Simply: A=bone which should stay | B=the other one | Vertex set=All | Mix mode=Add +2nd modif for other leg and apply both
Now disconnect bones to prevent its shifting during dissolving (simply uncheck connected checkbox in edit mode) And finally dissolve additional bone with Ctrl + X