Did J.K. Rowling give two or more unrelated characters the same first name in any of the 10 books?

Yes

• Agatha Chubb was a wizarding archeologist. Not to be confused with Agatha Timms, noted eel farm owner and non-professional gambler.

• Albert Runcorne was a Ministry of Magic employee. Not to be confused with Albert Boot, The former Minster of Magic (1747-1752).

• Arthur A. Levine was a noted publisher of books (including 'Fantastic Beasts and Where to Find Them' and 'Quidditch Through the Ages'). Not to be confused with Arthur Weasley, the father of Harry's best friend, Ron.

• Augustus Pye was a Trainee healer at St. Mungo’s. Not to be confused with Augustus Rookwood, MoM official and secret Death eater, or Augustus Worme, the editor of "Fantastic Beasts and Where to Find Them".

• Basil was the keeper of the portkeys at the Quidditch World Cup. Not to be confused with Basil Horton, the founder of the Comet trading company or Basil Flack, the former Minister of Magic (1752-1752).

• Bertie Higgs was a friend of Tiberius and Rufus Scrimgeour. Not to be confused with Bertie Bott, the noted confectioner.

• Bob Ogden was the MoM employee who visited Morfin Gaunt. Not to be confused with Bob Ollerton, co-founder of the Cleansweep broom company or Bob, the MoM employee that Mr. Weasley and Harry met in the elevator en route to Harry’s hearing.

• Cassandra Trelawney was a noted seer and grandmother of Sibyll Trelawney. Not to be confused with Cassandra Vablatsky, the author of "Unfogging the Future".

• Cornelius Agrippa was a noted alchemist with his own Frog card. Not to be confused with Cornelius Fudge, the former Minister of Magic (1990-1996).

• Daisy Pennifold was the inventor of the modern quaffle. Not to be confused with Daisy Dodderidge, the first landlord of the Leaky Cauldron.

• Damocles was the inventor the Wolfsbane potion. Not to be confused with Damocles Rowle, the former Minister of Magic (1718-1726).

• Dennis Creevy was a contemporary of Harry Potter at Hogwarts. Not to be confused with Dennis, the young boy who co-discovered Tom Riddle's seaside cave or Dennis, a member of Dudley's gang.

• Dugald McLivert was the head of a prominent wizarding family. Not to be confused with Dugald McPhail, the former Minister of Magic (1858-1865).

• Emeric the Evil was a known holder of the Elder wand. Not to be confused with Professor Emeric Switch, the author of "A Beginner's Guide to Transfiguration".

• Ernest "Ernie" MacMillan was a contemporary of Harry Potter at Hogwarts. Not to be confused with Ernest "Ernie / Ern" Prang, the driver of the Knight Bus.

• Frank Bryce was the Riddle family's gardener. Not to be confused with Frank Longbottom, Neville's father.

• Gladys Gudgeon was a huge fan of Gilderoy Lockhart and would often write to him in hospital after his (ahem) accident. Not to be confused with Gladys Boothby, the inventor of the Moontrimmer Broomstick.

• Harold Dingle was a contemporary of Harry Potter at Hogwarts. Not to be confused with Harold Minchum, the former Minister of Magic (1975-1980).

• Jacob Erland was a contemporary of Harry Potter at Hogwarts. Not to be confused with Jacob Kowalski, a no-maj baker who assists Newt Scamander.

• James "Jimmy" Peakes was a contemporary of Harry Potter at Hogwarts. Not to be confused with James Potter, Harry's father.

• Karl Jenkins is a contemporary of Harry Potter's son Albus at Hogwarts. Not to be confused with Karl Broadmoor, a beater for the Falmouth Falcons noted for his violent disconduct and repeated suspensions.

• Kevin Whitby was a contemporary of Harry Potter at Hogwarts. Not to be confused with Kevin, the little boy at the Quidditch World Cup who attempted to enlarge a slug or Kevin Broadmoor, the beater of the Falmouth Falcons from 1958 to 1969.

• Leonard Jewkes was the inventor of the Silver Arrow Racing Broom. Not to be confused with Leonard Spencer-Moon, the former Minister of Magic (1939-1948).

• Lily Moon was a contemporary of Harry Potter at Hogwarts. Not to be confused with Lily Potter, Harry Potter's mother.

• Marcus Flint was a contemporary of Harry Potter at Hogwarts. Not to be confused with Marcus Belby, whose uncle Damocles invented the Wolfsbane potion.

• Mafalda was a distant relation of Molly Weasley, not to be confused with Mafalda Hopkirk, an employee of the Ministry of Magic.

• Mary Cattermole was the Muggle-born wife of Reginald Cattermole. Not to be confused with Mary Riddle, the Muggle wife of Thomas Riddle or Mary MacDonald, a contemporary of James and Lily Potter, Harry's parents.

• Millicent Bulstrode was a contemporary of Harry Potter at Hogwarts. Not to be confused with Millicent Bagnold, the former Minister of Magic (1980-1990).

• Modesty Rabnott was inspiration for the inventor of the golden snitch. Not to be confused with Modesty, a young lady in the employ of Newt Scamander

• Montague was a contemporary of Harry Potter at Hogwarts and Slytherin Quidditch player. Not to be confused with Montague Knightley, the noted Wizard Chess Champion.

• Percival Dumbledore was Albus Dumbledore's father. Not to be confused with Percival "Percy" Weasley, the brother of Harry's best friend, Ron.

• Tiberius Ogden was an Elder of the Wizengamot. Not to be confused with Cormac McLaggen's Uncle Tiberius, a member of the Slug Club.

• Tom "the Barman" was the barman at the Leaky Cauldron. Not to be confused with Tom Riddle Jr., a dark wizard of some repute.

• Wilfred Elphick was the first man to be gored by an erumpent. Not to be confused with Wilfred the Wistful, whose statue can be seen in the hallways of Hogwarts.

• Wilhelmina Grubbly-Plank worked as a substitute Care of Magical Creatures teacher. Not to be confused with Wilhelmina Tuft, the former Minister of Magic.

• William "Bill" Weasley was the brother of Harry's best friend, Ron. Not to be confused with William "Bill" Ollerton, the founder of the Cleansweep broom company or William/Billy, the orphan whose pet rabbit a young Tom Riddle killed or William "Honest Willy” Wagstaff, purveyor of faulty wands and defective defensive charms or William "Willy" Widdershins, toilet vandal and part-time MoM spy or William "Will", the toad thief with whom Mundungus does business.

and last, but not least (and especially relevant given the question)

• Zacharias Smith was a contemporary of Harry Potter at Hogwarts. Not to be confused with Zacharias Mumps, who wrote an account of quidditch in the 14th Century.