Apologies if this has been reported previously - I cannot find this precise problem mentioned.
Doubling note durations of two bars with insert mode on causes overlap of doubled notes. All notes are in the same voice.
Welcome to the forum.
It appears that in your first bar you wrote the notes in the treble clef and shifted them down (with M) so actually the first bar is in a different voice than bars 2 and 3. This will cause the overlapping.
Thank you!! That indeed was the problem.