Chords will sometime cause a "Note already sounding" error (It's been bugged). Until it's fixed, here's a work around.
If you see
Chances are you'll get the error on the second C.
The simple fix is to change the length of the previous instance of the note (In this case, the C3) to match the length of the shortest note in the chord (the rest, in this case). I've tried this a few times and it seems to do the trick, without messing up the overall timing of the song.