The situation you describe in "Back to the Future Part II" where there are two versions of Marty McFly in the same timeline is not a mistake, but rather a deliberate plot point and an example of the "multiple timeline" concept within the movie series.
In "Back to the Future Part II," Marty McFly travels from 1985 to 2015 and encounters a future version of himself. This happens because Marty interferes with events in the past, creating an alternate timeline. In this alternate timeline, the future Marty exists alongside the younger version of himself who has just arrived from the past.
The "Back to the Future" series explores the idea that time travel can result in changes to the timeline, creating alternate versions of reality. When Marty interferes with past events, he alters the course of history, leading to different outcomes and the coexistence of multiple versions of himself in the same timeline.
This concept of branching timelines, where different versions of characters and events can coexist, is a fictional element within the "Back to the Future" storyline. It is not intended to represent a scientifically accurate portrayal of time travel but rather a storytelling device to create dramatic tension and explore the consequences of altering the past.
So, in the context of the movie series, the presence of two versions of Marty McFly in "Back to the Future Part II" is a result of the altered timeline caused by time travel, rather than a mistake.