Contact on messages are separate

Hello, It had just happened recently but when I had opened my messages, a certain contact had two separate messaging thread but both with the same name and picture, so now I have two of the same contact. I opened one and seen that it was up to date from the most previous messages but I had opened the other one and it only showed messages from about twenty days ago . The weird thing is, if I open my “contacts” app, they still appear as only one person, when I press their contact name and press “message” it shows two options “main” and “home”. I pressed both options separately and they both led to the different messaging threads. I’ve tried deleting one messaging thread and just leaving the other but when I delete it, it also deletes all those messages. So basically my contacts list shows them as one person but in my messages they are two different contacts from different times. I’m lost on how exactly I could move them both to one contact without deleting messages. The thread from 20 days ago holds photos and messages that are separate from the other thread but the one from 20 days ago won’t let me send a message, it automatically says “not delivered”