It takes two.
I've been thinking about identical twins in literature and I decided to make a list. There's no real point to it, except that I think there's a higher than usual number of redheads among them.
Tom and Pat Bartlett
Time For the Stars by Robert Heinlein
Hair: Not sure, but not red
Castor and Pollux Stone
The Rolling Stones by Robert Heinlein
Hair: Red, I believe
Sandy and Dennys Murray
A Wrinkle in Time interalia by Madeleine L'Engle
Hair: No idea. In fact, I'm not even sure they're identical.
Elladan and Elrohir, sons of Elrond
Lord of the Rings by JRR Tolkien
Hair: Dark
Brent and Stuart Tarleton
Gone With the Wind by Margaret Mitchell
Hair: Red
Fred and George Weasley
Harry Potter books by JK Rowling
Hair: Red
Jessica and Elizabeth Wakefield
Sweet Valley High sort of by Francine Pascal
Hair: Blonde
And they were the only girls I could think of.
Who did I miss? Looking for identical twins here, so please don't bring up the Bobbsey Twins.
[Edited to correct spelling of "Bobbsey"]
[Edited again to correct factual error that nobody called me on]

Parent Trap
I don't recall Padma and Parvati being twins, but I could have been imparting my own interpretation. They are not portrayed by twins in Goblet of Fire.
Re: Parent Trap
"Moon"..., "Nott"..., "Parkinson"..., then a pair of twin girls, "Patil" and "Patil" (SS 121) Dunno if they're ident, but definately twins. (And if Harry knows they're twins, isn't it probable that they're identical?
Re: Parent Trap
"Brothers and sisters usually go in the same Houses, don't they?" [Harry] said. He was judging by the Weasleys, all seven of whom had been put into Gryffindor.
"Oh no, not necessarily," said Hermione. "Parvati Patil's twin's in Ravenclaw, and they're identical. You'd think they'd be together, wouldn't you?"