Caliban, a character who's usually associated with the underground mutant group, The Morlocks, has shown up twice in recent X-Men films, played by two different actors - Stephen Merchant and Tómas Lemarquis. With X-Men: Apocalypse set in 1983 and Logan unfolding 46 years later in 2029, fans have been developing complicated flowcharts and theories to try and explain how these type of inconsistencies could actually make sense. Well, it seems the fans are paying more attention to the details than the film's producers and creative teams.
“It’s a funny, messy story of how so often these things are not as coordinated as everyone thinks,” Mangold told Nerdist's Victoria McNally in the video below. “I actually had written him into our movie, and they didn’t know [he was] in Apocalypse, and then they kind of wrote it in their movie, and they cast someone in their movie and I had not seen it and was working away on mine.”
So in short, the reason why Caliban appears in both movies is because no one was paying attention. It's these continual minor mishaps that persist and ultimately undermine Fox's efforts to build a cohesive cinematic universe for Marvel's mutant superheroes. Could you imagine if Fox were to attempt to weave in the Fantastic Four into the current X-Men continuity? On films that cost hundreds of millions of dollars to market and produce, it's a little mind-boggling that a mistake of this nature could occur.