Michael Jackson and Diana Ross.
Oh, wait. You said actresses, not vocalists.
I always get Tiffany Thiessen and Yasmine Bleeth mixed up.
Zsa Zsa Gabor and Ava Gabor
Richard Harris and Ian McKellen. Enough so that after Harris died it was reported several times that McKellen was the one to have passed on.
Heather Locklear and Heather Thomas!
Since actors are OK, how about Charles Durning and Brian Dennehy?
Leelee Sobieski & Helen Hunt.
Christine Lahti and Allison Janney
And Michael Douglas
Robin Tunney and Zooey Deschanel
Cate Blanchett and Tilda Swinson
Sophia Myles and Kate Winslet
Dick York and Dick Sargent. 
Looked But Didn’t Act Alike:
Margaret Sullavan and June Allyson
Barbara Stanwyck and Paulette Goddard
Wendell Corey and Tom Ewell
Look and Act Exactly Alike:
James and Edward Fox
Never Could Keep Them Straight:
Stewart Granger and Farley Granger
Thomas Mitchell and Grant Mitchell
Ron Livingston
and Kyle Chandler
Lara Flynn Boyle and a broomstick.
Seriously, though, I can’t tell the difference between Naomi Watts and and Scarlett Johansson
){kind=link}
Kim Catrall and Theresa Russell?
Do you mean Cate Blanchett?
The recent death of Jane Wyatt reminded me of how similar her name is to that of the apparently still-living Jane Wyman.
Then there’s the Vanessa Williams who was Miss America and the one who wasn’t.
It took me a long time to figure out that Laurie Holden was not Lauren Holly.
Tom Sizemore and Michael Madsen
One or both of them are often confused with the singer Jewel, as well.
Many years ago, before Mad About You turned Helen Hunt into a household name, she and Ellen Barkin were frequently confused for one another (I recall at least one interview with Ellen where she said it was a common occurrence in her life). Now, as she’s aged, Ellen looks a lot more like Angie Dickinson to me.
