Midi notes deleting when editing and writing in notes

[SeventhSounding]

Whenever I edit MIDI notes in the MIDI editor, if I copy and paste a note (mainly when editing drums) if the note appears under another note, it causes the note above to delete. Is there any way to fix this? Or is there a reason it does this and there is another way to go about it? I mainly have this problem when editing my what I record from my electronic drum kit. I use Addictive drums 2 and send the midi signals from my Alesis kit into Pro Tools.