There’s an easy way to tell the difference–Chris Rock is the one who is actually funny and entertaining.
Brian Dennehy (F/X), Charles Durning (Dogday Afternoon) and Kenneth McMillan (Ragtime) though not so much the last guy since he died in '89.
Ashley Judd and Angelina Jolie.
Elijah Wood and Tobey Maguire (one’s Spiderman, the other Frodo).
George Hamilton and James Brolin.
Michael Keaton and Michael McKean (just the names, they look totally different, i.e. one’s Batman the other Squiggy!)
And especially Ellen Burstyn and Louise Fletcher. One of them was in The Exorcist and the other Cukoo’s Nest…
Welcome, Comcat!
I’d like to second syncrolecyne on Gary Busey and Nick Nolte – and Angelina Jolie’s father, whathisface, is the third member of that unholy trio. Jon Voight.
Also – Matthew McConneghy and Matthew Moline. Which one is the bongo player, again?
Stephen Dorff and Skeet Ulrich.
Aside from looking like twins, they both had some “buzz” at around the same time. Now they both equally have no buzz, making it impossible to tell them apart anymore.
what the holy hell!!! is that my password at the top of my last post?? I didnt put that there!
can you guys see it? or am I going crazy?
ok never mind i checked my paper with my passwords and thats not it, I must have typed that there out of habit becouse thats a characters name on a stupid game I play. 
I was watching Glass House and until I realized what it was I could have sworn LeeLee Sobieski was a young Helen Hunt, only with much better boobs.
No, one’s Batman and the other one’s Lenny. 
For me, it’s Jill St. John, Susan Saint James and Lili St. Cyr.
I have to remember that it’s Tiffany Case of “Diamonds are Forever” then Kate of “Kate and Allie” then an ancient stripper.
Gary Busey and Nick Nolte.
Something to do with craggy lookin’ ne’er do wells!
I think they are cast as similar character types which really doesn’t help!
(or am I imagining it? Heelp!)
All seriousness aside, I’ve got a bad Geena Davis/Julia Roberts thing going on.
Cate Blanchett and Kate Winslett - name wise. Not visually.
Yes, either would do. Both would be better.
Lenny & Carl:
This one is easy. Kate Hudson will show her boobies!
On the guy side, I think Richard Gere and Warren Beatty are the same person.
Sure you can…Ashley’s taller, has wavier hair, and has slightly larger breasts. Mary Kate’s hair is straight.
Carol Burnett and Mary Tyler Moore for me.
On SNL, Jimmy Fallon, Jim Bruer, and Chris Kattan were like one big mixup.
Joan Crawford and Bette Davis. It didn’t help when I saw Whatever Happened to Baby Jane?. 
I’m looking & I’m amazed no one said Bill Paxton & Bill Pullman (I think I finally got it straight- Paxton has a happily demented look).
and YES- Vicki Lewis & Kathy Griffin!
Matthew McConaheigh & the guy who starred with Reese in SWEET HOME ALABAMA- and maybe John Corbett (NORTHERN EXPOSURE, SEX & THE CITY, MY BIG FAT GREEK WEDDING)
While not a mixup exactly, when I saw John Corbett on Sex in the City, I assumed it was Edward Burns. I didn’ tknow John Corbett existed.
That was just… disturbing.