OnTriggerStay2D Updating

i have a problem with OnTriggerStay2D

void OnTriggerStay2D(Collider2D other) {

if (other.transform.position.x == 0.0f) {
if(other.transform.position.y != 0){
go.Add (other.transform.position.y);
}
if(other.transform.position.y == old){
go.Remove(other.transform.position.y);
}
old = other.transform.position.y;
}
i need to add the positions of all objects is on triggers to a list but the problem that is adding the all position the ,old position and the new position of the objects is ontrigger, so what i need to do is when any objects is ontrigger and if he change his position mean move so, save the position of this object and delete his old position i need to do this for all objects is ontrigger

thanks .

 

some helps and thanks