Video games are constantly being compared to one another. Sonic's a faster Mario, Battlefield is a more realistic Call of Duty. The comparisons never end. Sometimes they foster rivalries that help both sides thrive. Other times a game is actually just a shameless clone of the other and lawyers get involved...
The more successful that something is, the more likely it is that it'll get copied. Think about how many first-person shooters showed up after Call of Duty took off. A certain genre that involves 100 players is reenacting that same situation now.
If you want to see what an undebatable rip-off looks like, just search up the Chinese version of practically any game out there. Final Combat is just Team Fortress 2 with different names, Overwatch has multiple Chinese rip-offs (and just rip-offs in general) like Hero Mission and League of Titans.
What we're about to go over are some of gaming's most infamous (and successful) rip-offs. The ones that are famous enough to have reputations beyond just being copycats. Some of them will be good, some will be bad, but all of them will be unoriginal.