Katy Perry and Zooey Deschanel, too.
I know someone who thought that the star of Who Framed Roger Rabbit was Danny DeVito instead of Bob Hoskins after she’d finished watching it.
Samuel Jackson and Larry Fishburne.
I’ve always had to do the same for the two Hurts.
You mean today? My poor old brain does this all the time.
Leslie Nielsen and Robert Graves were both in Airplane! but i can never remember which one played which role.
If I had to guess Nielsen was the doctor and Graves the pilot.
You’re correct by last name, but it was Peter Graves. The cast did also include Robert Hays, so I can see where first names got scrambled. 
Not for another two hours.
I go the other way: I grew up not noticing such things on our low-res B&W 19in TV, but now, when watching old TV shows, I’m amazed how many people have yellow teeth. Smoking! It’ll ruin your looks, kids!
And crooked teeth. I’m one to talk. I came from the generation where orthodontics was expensive. Now everyone has perfect perfect teeth. It’s like some Twilight Zone episode.
I can guess again in two hours?
Back to the OP, I get Cary Grant and Gregory Peck confused. Side by side, of course, they don’t look at all alike, but they both star in romantic movies with Audrey Hepburn, and they both were in Hitchcock thrillers. So I really have to think on random films. “Which was in The Enemy Below?* Which one is Spellbound and which one is Suspicion?”
*See what I mean! It should be Peck!
Matt Damon and whathisface… the one in Saving Private Ryan… ah no that was Matt Damon too, Bourne! ah no Matt Damon too, God dammit.
googles “Matt Damon lookalike actor”
Mark Whalberg!
Cary Grant, Gregory Peck AND Clark Gable are all the same guy to me.
When 2 Broke Girls first aired, I assumed Beth Behrs was the actress who played Max and Kat Dennings was the one who played Caroline, because “Beth Behrs” sounded more like a “brunette” name and “Kat Dennings” sounded more like a “blonde” name.
That’s funny. They are all completely different to me. All they share is an era. To me they look nothing alike and have very different acting styles.
Sorry: that was Clark Gable.
You confused Marilyn Monroe with Clark Gable?
Watch the ST:TOS episode “Let That Be Your Last Battlefield,” where the camera zooms in on the mouths of Shatner, Nimoy, and Doohan as they issue the self-destruct order. It’s enough to make a dentist cry and an orthodontist swoon. 
Robert Mitchum was in The Enemy Below, Cary Grant was in Suspicion and Destination: Tokyo (another submarine flick), Clark Gable was in Run Silent, Run Deep, and Gregory Peck was in Spellbound.
Well, the former is Clark Kent, and the latter is Bruce Wayne.
Clark Kent and Cary Grant look nothing alike. They can’t be the same.
And no, it’s not the glasses…
“Christopher Reeve Based His Superman on This Screwball Comedy Performance … Most obviously, the look of Clark Kent is a perfect recreation of David Huxley. Slick black hair that flops down perfectly in the front, thick black-rim glasses, dapper suits that emphasize the impressive length and height both men had, and a knack for pantomiming discomfort and anxiety. As Clark Kent, Christopher Reeve perfectly mimics how David will raise his hand to make a point, and then immediately lower it in resignation once he’s been cut off by someone else…”
https://collider.com/christopher-reeve-superman-cary-grant-bringing-up-baby/