> Keira Knightley & Natalie Portman
Wait, which one is the "off-brand" here?
> Bryce Dallas Howard & Jessica Chastain
I must confess I used to confuse these two!
Another one you didn't mention: until relatively recently, I thought actors Elias Koteas (Exotica) and Christopher Meloni (CSI: Special Victims Unit) were the same person!
Bryce Dallas Howard & Jessica Chastain
Selena Gomez & Lucy Hale
Amy Adams & Isla Fisher
Keira Knightley & Natalie Portman
Not to mention that anytime an actor ages out of the early adulthood age range, a lookalike begins to play the same roles.
If you can't think of any examples, it's probably because you haven't been able to tell them apart yourself.