Why are texts going to a random stranger and myself?

[AC Question]

First of all, I apologize if this has been asked before. So today my girlfriend texted me (she has a Galaxy S6 and I have a Nexus 6P) and somehow both me and some stranger got the same text. It was just a single conversation between me and her, but this other person also got the same text from her and when they replied it came back to her and said it was from me. Again she replied to the message thinking it was me and again I got the response and so did the stranger. What in the world is going on? All 3 people were confused and now my girlfriend is afraid to text or call me.