The great thing about X-Men comics is how each character possesses a unique mutation that makes them special. It's the intermingling and opposition of these unique power sets that make each battle so intriguing. The powers represent an integral component of each delicately crafted personality and delineate characters that can thrive for generations.
That uniqueness highlights a central theme of the X-Men mythos: the chaotic and inimitable process of random mutation. Presenting evolution through the prism of individual perception builds a powerful allegory that has drawn in readers for over 50 years. That's why it's so important that no two mutants are alike... at least until a writer runs out of gas and throws in the towel.
Over the years, X-Men writers have hatched some amazing characters and then followed them up with knock-offs and carbon copies that are in a word... redundant.The expansion of the mutant population has at some points inferred the existence of thousands of mutants, so some overlap is inevitable, but it's no accident that some of the worst offenders mimic Marvel's most popular characters. Whether they be clones, descendants or just sloppy writing, these are the most redundant X-Men characters of all-time.