When a key is pressed, it has to be released before one can press it again. While this seems completly obvious, let's not forget that music sheet notation where made for humans, not computers. And as a consequence, processing music sheet requires some care.

For example, on the following music sheet:

the first la key is meant to be pressed when starting the music, and released at the exact same time the user should press the sol key. In between tough, there is the grace la key which should be played after the "big" la and before the sol key are pressed. That is, the user should press la while it is already pressed. Any human will understand that to press the la the second time, you have to release it first and will instinctively do it. Therefore the program will look for these situations where a key is meant to be pressed while it was already pressed. When this happens, the first press key will be changed so that it will be released when the second one is meant to be pressed.