Problem with iMessage (2 different people answering from the same number)

So my friend got a new phone/number today, he has a galaxy and I have an iPhone 5s. For about an hour I was able to text him just fine and as I normally would. Then all of a sudden, when I tried to text him it switched over to iMessage. So I sent the message using iMessage and asked if he had decided to get an iPhone instead and if he had just activated iMessage. Someone else that wasn't him began replying through iMessage and it appeared as the same number. So I turned off iMessage and texted my friend, and it went through to him just fine. I turned it back on and again when I tried to send him a message it went through iMessage and sent to the other person. So whenever I had iMessage on, it went only to the other person, and whenever I turned it off, it went only to him, even though whenever each of them replied it appeared as the same phone number for both. My friend said he texted his neighbor, who has an iPhone, and didn't have any issues receiving messages with her. Unfortunately he doesn't know anyone else with an iPhone to text/test this further, so so far it seems like it's just happening with me. I decided to ask the other person if they had the same phone number as my friend. They said yes but that they're getting a new phone and new number soon so that might be why? I really haven't heard of this before. Has this happened to anyone else? Anyone know if the problem is on my end or his, and if there's anything I can do to fix it? It gets rather annoying to keep turning my iMessage on and off just to reply to him. Thank you.