Some 30 years ago, someone challenged me to tell the difference between Pepsi and Coke in a blind taste test. After taking several sips, I could eventually tell that one tasted just a little bit sweeter, more sugary, and the other one tasted just a tiny bit more... "dark" is how I put it at the time. (Note that I was using that word to describe a flavor, not a color. I do not have synesthesia, that's just the best word I could find to describe the subtle taste difference). I guessed that the slightly-sweeter one was Pepsi, and I turned out to be right.

Thing is, since doing that taste comparison where I alternated sips several times between the two, I've consistently been able to tell if a drink was Pepsi or Coke. So while they are very very close, they are distinguishable to some people, if those people have trained their taste buds. (Or at least they were up to about 10 years ago, I don't know if they've changed the flavor in the past decade because I practically quit drinking soda at all once I got serious about maintaining a healthy weight.)