Cloning as a motif is very widespread in the pages of Marvel Comics. Characters get cloned rather effortlessly when the plot demands it, by villains or other third parties who, understandably, want the powers of the superheroes for themselves or whatever logic it is that villains run on. It is a trope comic book writers fall back on often to introduce twists into the story, but is something of an overused concept.
Most clones that are introduced are quickly forgotten until the next story arc that puts the focus on them, but there are exceptions like X-23 who have managed to make a name for themselves and grown beyond the identity of just being a clone.
Max Eisenhardt (Magneto) - Joseph
Joseph first appears in X-Men Vol 2 #46 (1995) and soon after joins the X-Men temporarily, initially thought to be an amnesiac Magneto despite being decades younger than him. Then, subsequently, in issue #86 it is revealed that he is actually a clone of Magneto created by one of his former allies Astra to further her own ulterior motives after she had rescued his life in an incident.
Joseph sacrifices himself in a fight against the real Magneto, dying a hero. Years later, he is brought back again by Astra, with the roles being reversed this time, Magneto on the side of the good guys and Joseph as a villain.
En Sabah Nur (Apocalypse) - Evan Sabahnur (Kid Apocalypse)
Apocalypse is one of the best X-Men comic villains, with a track record of dying and then being resurrected again quite a few times. In one such instance, he was brought back in the form of a young boy with no past memories and essentially a blank slate. Fantomex kills the child on a mission in Uncanny X-Force #4, but also secretly takes a blood sample from the body which he uses to create a clone.
This clone is then brought up by Fantomex to be a good person, an antithesis of everything Apocalypse stood for, and he ultimately proves his heroic nature after sacrificing his life in a fight against the villain Omega Red.
Jean Grey (Marvel Girl) - Madelyne Pryor
During the 1980s, at a time when the actual Jean Grey was dead, Scott Summers met and fell in love with a woman named Madelyne Pryor who bore an uncanny resemblance to Jean. The two married and were happy for a time, and even had a child together (Cable). But as revealed in the pages of Uncanny X-Men Vol 1 #241, Madelyne was actually a clone of Jean Grey made by Mister Sinister specifically to fall in love and have a child with Scott.
Madelyn would go on to become one of the most underrated X-Men villains as Goblin Queen, and later on the Red Queen.
Nathan Summers (Cable) - Stryfe
Even disregarding the fact that he is the son of a clone himself (Madelyne Pryor), Cable's history is still convoluted enough to confuse anyone. After being infected at a young age with the Techno-Organic Virus by Apocalypse, he is taken 2,000 years into the future by an alternate reality version of his sister where he is brought up.
While there, a clone of Cable is created, something only comic book fans will know, who is adopted by the Apocalypse of the future. Stryfe later goes on to become an X-Men villain responsible for, besides other things, the spread of the dangerous Legacy Virus that killed many mutants.
James Howlett Logan (Wolverine) - Laura Kinney (X-23)
Wolverine's powerful healing factor makes his genes a valuable commodity in the eyes of many, thus making him a constant target of cloning attempts in the comics, and X-23 represents the best of them.
Trained to kill from birth and abused from a young age constantly, it is for the best that writers have allowed her to settle into a more traditional superhero role now. Neither Logan nor Laura fit into the standard father-daughter roles due to their very nature, but the two still make for a wholesome pair in the comics.
Laura Kinney (X-23) - Gabrielle Kinney (Honey Badger)
Taking the cloning shenanigans related to Wolverine even further, even his clone Laura Kinney has a clone in the comics in the form of Gabrielle Kinney. Gabby first appears in All-New Wolverine #2 besides many other clones of X-23, dubbed The Sisters, created by Alchemax to be the perfect killing machine just like Laura.
After escaping their captors together, Laura and Gabby went on to live together as sisters and had many adventures in X-23's solo comic runs. Gabby has Laura's healing factor but only a single bone claw on each arm, and she is unable to feel pain.
Emma Frost - The Stepford Cuckoos
First introduced in New X-Men #118 (2001), the psychic sisters Stepford Cuckoos were an interesting addition to X-Men comics and their powers mirrored that of Emma Frost, one of the strongest telepathic mutants. But as revealed during the limited series X-Men Phoenix Warsong (2006) their relationship goes deeper than just having blonde hair and similar powers, and in reality, the Cuckoos are part of a huge batch of clones created by the villain Sublime from Emma's genes.
Out of the thousands of clones created, only five were made active, and now the three that remain have become a regular part of the X-Men lineup.
Fantomex
The mysterious past of Fantomex is a big part of his appeal, and it's rare for him to show his face even, but what is known about the character is that he was created by the same program that made Wolverine, Weapon Plus. In a fight against the Brotherhood of Evil Mutants, Fantomex is killed in Uncanny X-Force Vol 1 27 (2012) while trying to save Psylocke.
In the final issue of the series, he is brought back but due to an error caused by Deadpool's interference, his three brains are split into three different clone bodies, creating a Lady Fantomex and Dark Fantomex.
Natasha Romanoff (Black Widow)
Natasha Romanoff died at the hands of Evil Captain America during the events of Secret Empire. In the consequence of her death, it is revealed that the Red Room, the organization that trained Black Widow, keeps clones of all of its most powerful members in the event of their death, and that is how Natasha is brought back.
Clone Natasha is given all the memories of the original Natasha, and after discovering her origin as a clone she turns against the Red Room and destroys all the other clone copies, as explored in the Red Ledger story arc.
Peter Parker (Spider-Man) - Benjamin Reilly And Kaine Parker (Scarlet Spider)
There have been multiple major clone-related events in Spider-Man's long history, which only goes to show how tangled he is as a character with the concept. Chronologically, Ben Reilly is the first one to appear, in 1975's Amazing Spider-Man #149, but later comics reveal that the imperfect clone who would go on to call himself Kaine Parker was the first to be created.
Ben was also the first to don the moniker Scarlet Spider, in his attempt to distinguish himself, and currently, Kaine goes by that name. After years of trials and tribulations, the three Peter Parkers have managed to make their own place in the Marvel Universe.