Recently discovered that my boyfriend went to his family Christmas gathering with his ex while he was courting me. what should I do?

Been with my current boyfriend for close to a year now, only came to know recently that during the period when he was courting me, he still went to a family Christmas gathering with his ex. He said they weren't together already but kept that arrangement because they did not want his parents to know (afterall they had been dating on/off for 7 years) and question him about it. It's hard for me to fathom how can he be flirting with me while being in a family gathering with his ex. I'm happy with him so far and it doesn't seem like he is still talking to his ex ever since we started dating, but I'm not sure if this is something I shld let pass and move on or shld I break up because of this?

Recently discovered that my boyfriend went to his family Christmas gathering with his ex while he was courting me. what should I do?
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