Oh! I Always Get Those Two Mixed Up!

Andy Garcia and Mark Strong. Seriously, I thought these two were the same person until very recently.
http://www.bbc.co.uk/pressoffice/images/bank/programmes_tv/drama/300longfirm_strong3.jpg http://www.boston.com/ae/sidekick/blog/Andy_Garcia_XX.gif

Gogol Bordello/Dresden Dolls

Ballad/Silence

Taylor Swift / Hanna Montana

Rikki Fataar/Rikky Rooksby

Americans have heard of Rikky Rooksby?

(I know he's written a couple of books but they're in a pretty over-crowded field and I know the name mostly for him writing for many years for a magazine that was handed out free in music equipment shops across the UK)

(PS seems a good guy and have probably learnt a few things from him, so this is not intended as an insult)
(PPS Kompakt act The Field's real name is Axel Willner, if you want to add anyone else to the confusion whirlpool)

Lightning Bolt
Lightning Seed

for a sec I thought you meant Val Kilmer, so Val Wilmer/Kilmer

Axl Rose
Hulk Hogan

mitchell and webb, mitchell and ness

Cardiacs ≠ Cardigans

Lilith
Judith

Probably not the thread for this, but
the "h" that seems to have gone missing from John Entwistle's name and appeared in Pete Townshend's.

Sean Wilentz (historian)
Sean Wilsey (memoirist, friend of Alex in NYC)

B'nai B'rith / Baha'i

Key Cars
K-Cars

Clan of Xymox
Xmal Deutschland

Roger Williams / Roger Miller

Michael Alig
Michael Azerrad

Wheatus and Weezer. Both were punkish American rock bands that had a novelty hit song in the 90s, never could remember which was which.

Ween and Weezer for me.

Sheila E / Sheena Easton

Lucinda Williams / Victoria Williams

Gil Evans (Miles Davis arranger) vs. Bill Evans (Miles Davis pianist) vs. Bill Evans (Miles Davis saxophonist)

Jeremy Vine and Jeremy Kyle.

Jeremy Vine and Jeremy Kyle.

are brothers, by different father.

Noel Coward / Cole Porter

Boyz 2 Men / They Might Be Giants

I have no explanation for this, but I do mix them up. I'm not sure if I've ever heard either one's music though.

Fiona Apple / Jewel

François Bayle / Frankie Boyle.

Cymande / Mandré / Mandrill

andres malraux, gide and brink

haha jim

Scissor Sisters
Switchblade Symphony

Boyz 2 Men / They Might Be Giants

I have no explanation for this, but I do mix them up. I'm not sure if I've ever heard either one's music though.

this is amazing.

for years I wondered how this dude had time to come up with so much stuff considering he was only famous for being tall and died pretty young

http://en.wikipedia.org/wiki/Robert_Wadlow

I was confusing him with

http://en.wikipedia.org/wiki/Ralph_Waldo_Emerson

Irving Azoff/Samuel Z. Arkoff

Warren Ellis/Garth Ennis

It has only recently come to my attention that Evan Parker and Andrea Parker are not the same person. o_0

Warren Ellis/Warren Ellis

Ha. That's like
Christopher Priest/Christopher Priest

It's an easy mistake to make!

Sons of Adam/Children of Men
tempera/tempura

Tony Moreno/Tony Marino (drums/bass respectively)

Frank Rich
Frank Bruni

Dean Jagger/Dean Jones/Dean Stockwell

Neil Morrissey and David Morrissey.

Kenneth Fearing/Kenneth Anger
Roy Ward-Baker/Roy Thomas Baker/George Roy Hill

Record labels Matador and Matinee

I don't get these mixed up, but watching Licensed to Kill last night I was amazed at how not only does he sound like Julian Barratt, Dalton also had very similar eyes.