Keep in mind too, especially when editing.... each midi note has an on signal and an off signal in the note.
If you have 2 notes ...exactly the same pitch....and lets say one is a whole note and the one following it is a whole note.... and you edit the first one to be slightly longer..... so that it now ends slightly after the second note starts..... the second note will never sound.
On the staff, it will look normal..... but in the world of midi.... you will have an on signal.... followed by another on signal on the same note followed by an off signal and another off signal. Only the first note gets played.
This used to blow my mind until I figured it out one day looking at the note duration.
It will not happen if you are playing the notes in from a keyboard. It can happen easily if you start to edit the notes manually by dragging notes around.