Robin Williams and Bono . Click on the double pic.
Sorry for the bump, but it just occurred to me Trey Parker of South Park fame looks just like the younger brother of Ray Stevenson who plays Pullo in the HBO show Rome.
Thats Jack Brown 
Howard K. Stern and Glenn Quagmire.
What? Nobody else has noticed? Go ahead, check. Amazing isn’t it!
GWB and younger Johnny Carson.
Oh, hey. The David Cassidy thread just jogged my memory.
I’ve always thought that Nicholas Lea who played Krycek on The X-files looked a fair bit like Shaun Cassidy. Or at least he looks more like Shaun Cassidy than David Cassidy does.
The resemblance between former baseball player Darryl Strawberry and Dino (the pet dinosaur from the Flintstones) was uncanny.
First remarked upon in Spy magazine if I remember correctly.
Yikes!!! I can only say to the 1st part, Nick Nolte should be genuflecting to his dentist!!! :eek:
Robert Sean Leonard (Dr Wilson on House) and
Thomas Gibson (Greg from Dharma and Greg and Hotch from Criminal Minds).
I had no clue they were different people until just recently.
Spy magazine caught this about 15 years ago.
How about Howard K. Stern and The Count.
Richard Jeni and Kevin Pollak.
American funnyman David Cross and New Zealand funnyman Jemaine Clement (on R).
I absolutely cannot tell the difference between Tom Jane and James Purefoy. I can sometimes tell Christopher Lambert apart from the other two.
Tom Jane and Christopher Lambert could be brothers or twins. Purefoy looks different to me though.