A lot of people seem to get Amy and Isla confused, but it's actually quite easy to tell them apart, because aside from their hair and cutesy demeanor, they really are not that alike.
I for one think that the main reason everyone gets them confused is because they are both red-haired. Lets see what you think when Isla is a blonde:
Isla in Scooby-Doo!
Still think they look alot alike? Let's research it further:
Amy has large, bright blue eyes; Isla's dark brown eyes are smaller.
Amy's nose is more pointed than Isla's button nose.
Amy is fair-skinned while Isla is more tanned.
Isla has a round face, while Amy's is more heart-shaped.
Hopefully now you can tell them apart! :)