Two voices each with an accidental
1) A note in Voice #1 with an accidental (e.g. Bb in a base key signature of C) is added to a stave.
2) A second note (same value, and also Bb) is added as Voice #2 in the same stave.
3) On the screen, and in printed copies, the notes are merged into one note body, but the accidental to the left is repeated, and therefore appears on the screen (and print) as a "double flat". To a musician, the quick indication is that the two notes are to be played/sung as the base note but taken down two semi-tones, rather than each of them down one semi-tone.
I recommend that (as is normal for commercial scores - where only one accidental is shown), MuseScore software be modified to show only the one accidental if both notes are the same.
If the two notes are different (i.e. one has an accidental, and one does not), the two notes should be separated and shown side-by-side - as is the case if the two notes have different values.