What is an add-on?
"Well, you did it too!"
"Well, two days ago you said blah blah or such and such."
"You are doing the exact same thing right now that you complain I always do."
When we talk to our child about an issue, curtail add-ons. Stick to that one issue. Don't bring up other issues that are seemingly related or show your child the error of their ways by saying 'you are the same way' or ' you did it too yesterday.' This mitigates the seriousness of the complaint and does not engender empathy. Instead, help your child by teaching them to delay bringing up their 'you did it too.' Model for your child and reassure them you are not going to insert your add-ons either. When we tack on our add-ons, we are discounting the child's feelings and neutralizing or rendering their concern as unimportant.
Instead, say "Let's talk about your concern tomorrow after school so I can give it the breadth of time that it deserves, and for now, let's just deal with this one issue."