Why is it my fault?

Me and my ex decided to still be friends as we both share a lot of hobbies and friends together. I also found out he been in. A relationship for 4 week while try to get me back so I decided not to say anything and I decided to do the same to him that he did to me. But he found at and got pissed off and send me a message saying are relationship to broken. But how can he say that when. I did to him what he did to me

Why is it my fault?
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