system
Given the following post, respond as if you're commenting on Reddit:
human
On Back To The Future 1, marty wakes up in their house but in a completely different timeline, what happened to the Marty that was originally living in that timeline?

On Back To The Future 1, marty wakes up in their house but in a completely different timeline, his family is different and his house is different... when he went outside, he saw his girlfriend and told him that "it's like you haven't seen me for a week." indicating that there was a Marty that was there in that timeline. 


So what happened to him and where was he when the first marty woke up in his bed?
gpt
Back to the Future uses a single-threaded time travel system, though with a very special difference: the "ripple" effect has a speed.  There is only one timeline, but changes made in the past take time to ripple in the future (notably seen when Marty doesn't immediately disappear when he stops his parents from hooking up, and the fact that Old Biff was able to re-enter their version of 2015 after delivering the almanac to 1955).

Therefore, Marty was there in that timeline, until 1 AM that night when he met Doc and went on their wild ride.  He returned later that morning, technically only "missing" a few hours in 1985.  It's likely that when he arrived, things were still as they were when he left (or at least similar), and during the hours he slept or whatever he did before the next scene, the universe changed around him.

There was no "first marty".  The only time duplicates are possible is when you travel from two different times to the same target time - such as in BttF2.

EDIT: to address her reply, that was a response to how odd he was acting since he was very excited and affectionate when she saw him, because he had been away for quite a while in "true-time" (the measure of delta time spent by his person in 1955)