Basic Trope: A character is killed but the actor returns as his hitherto unseen twin sibling.
Straight: Alex is Killed Off for Real. In the next episode, Alex's long-lost twin brother Bob is introduced and continues to play pretty much the same role.
After a Kill 'em All, all killed characters are revealed to have long-lost twin siblings.
Or, the twin dies, and it turns out there were triplets...then the triplet dies and it turns out there are a whole ton of backup siblings.
Bob has vats of clones ready to replace him in case he dies.
Downplayed: Alex is Killed Off for Real. In the next episode, Alex's long-lost twin brother Bob is introduced and makes a few guest appearances from then on.
Justified: Bob was innocuously mentioned or even introduced ten episodes earlier and other characters specifically seek him out and convince him to take over Alex's duties.
Inverted: Alex resurrects his dead twin brother Bob, then takes a Long Bus Trip.
Bob is actually an impostor who took on Alex's appearance to infiltrate the circle of his friends.
Alternatively, It turns out that Alex has faked his death, and that Bob is actually him.
Alernatively, Bob shows up... And, aside from looks, is nothing like Alex.
After getting to know Alex's friends, the impostor Bob decides to become the mask.
Alternatively, it turns out that Alex was Bob all along, got tired of acting like his twin, faked his death, and reappeared as himself.
Parodied: Alex seemingly dies but returns after Bob jumps in, kicking off a heated argument on who is the real Alex now.
Zig Zagged: Alex dies and is replaced by his twin sister as a favor to the actress playing his part.
Averted: Alex has no twin siblings.
Enforced: "But he signed a contract with us for the entire season! You can't just kill him off like that! Find a way to put him back in!"
Lampshaded: "Ah, you must be Alex's twin brother. Nice to meet you. I guess there's no need to tell you that He Didn't Make It."
Invoked: "He's Dead, Jim." "Check his family records for any unaccounted for twin siblings."
Defied: Hearing about Alex's death, Bob blows town and is never seen again to avoid associating with whatever business Alex was in.
Discussed: "Sir, I know what you are thinking but if Alex did have a twin, don't you think he'd tell us?"
Conversed: "He is a main character, duh... at least, they didn't pull out a Disney Death."
Deconstructed: Alex's friends find Bob whose personality is nothing like Alex's, and an attempt to substitute him with Bob ends badly.
Reconstructed: Alex's friends find Bob whose personality is nothing like Alex's but he learns to do what Alex did in the group and they learn to like him for what he is.
Plotted A Good Waste: Alex died in a way that was noble and deeply affecting, so the audience views the attempt to "bring him back" in the form of Bob as a cheap trick which sullies their memory of the character.
Played For Laughs: Though physically identical, Bob is nothing like Alex, forcing the main cast to go to ridiculous lengths to cover for his mistakes and/or explain his changed personality to those who knew Alex well.
Played For Drama: Alex's girlfriend cannot accept Bob but is forced to appear together with him to maintain the secret of Alex's death.
The main page is dead? Don't worry, a Backup Twin will show up soon enough.