iMessage splits one contact into two message threads

I have an iPhone XR that I haven’t had any issues with. Today, it died so I plugged it in. Once it was recharged, I got two messages from a contact. I haven’t had this issue before, and both messages were sent from the same device (his iPhone 11.) However, my phone somehow split the conversation into two threads: one with his messages and all of our old messages that I could not send anything to him in, and another which didn’t contain our old messages and I could send texts to him, but all of the received messages would go to the other thread. I have no clue what happened or how to fix this, it’s super annoying to have to exit one thread with his message so I can then reply. Can anyone help me with this?