Don't get too worked up about it, just correct her and say you prefer to be called Deborah rather than go on the offensive, could be that she knows a Debbie and so just uses it automatically, unless she is using it in spite, and you have said nothing to imply this, would be silly to start calling her other names.
I have an elderly neighbour who keeps calling me Bridget, it isn't my name but he is getting forgetful so have stopped correcting him and now most of the neighbours call me that on occasion as a joke. I don't mind because I don't actually like the name I was given whether it is shortened or not.