Don't believe me?
- Kitty Norville (Carrie Vaughn's novels)
- Kate Beckett (Castle)
- Ekaterin Vorkosigan (the Vorkosigan saga)
- Katnis Everdeen (Hunger Games)
- Catherine Earnshaw (Wuthering Heights)
- Catherine Willows (CSI)
- Kate Austen (Lost)
- Kate Lockley (Angel)
- Kate (Taming of the Shrew)
- Kat Stratford (10 Things I Hate About You)
I'm sure there are many, many more but that's all I can think of right now. It's nearly as bad as leading women with red hair.
Of course, male characters don't escape the naming convention thing either. Remember than Jack is a diminutive of John, and…
- Jack O'Neill (Stargate SG-1 and other)
- John Sheppard (Stargate Atlantis)
- Jack Harkness (Torchwood)
- John Winchester (Supernatural)
- Jack Sparrow (Pirates of the Caribbean)
- John Locke (Lost)
- Jack Spratt (Jasper Fforde's novels, though he's alluding to nursery rhymes and probably doesn't count)
- Ianto Jones (Torchwood)
- Ivan Vorpatril (the Vorkosigan saga)
- Johnny Mnemonic (movie of the same name)
And again, I'm sure I'm forgetting people. Anyone want to weigh in with more characters? Other names this happens with? Or an explanation of why we get so many repeated names?