I've known quite a few sets of identical twins who I could tell apart on sight. Once I'd known them long enough I found it easy. In fact, I remember temporarily forgetting that these two girls I knew were identical twins, because their subtle differences became quite noticeable over the time we were friends.
To the point where I couldn't imagine myself getting the two mixed up anymore. It started to become strange to me that anyone would, so I would remind myself of how I used to do the same when I was first getting to know them. One time another set of identical twins tried to trick me by acting like the other, but I also knew those two well enough that the trick failed.
"Hey, how are you?"
Me: "Oh, you've swapped your hair clips".
"...I told you it wouldn't work!"
"No, she just knows us too well, it'll work on someone else trust me".
They'd switched to different sides than usual, and swapped hair clips. Each twin wore a different hair clip to differentiate themselves from the other. Typically they kept this consistent unless trying to fool someone. Same hair length and style except for parting. However, whilst they'd both attempted to copy each other's hair parting it was slightly off from what it typically was, and their body language differed enough that I could tell the twins apart simply by how they stood. I notice these things, you have to try harder than that to fool me.
However, I can certainly be clueless in other areas.
I think it's interesting how the longer I've gotten to know people who are identical twins, the more obvious their differences become. Have you found this to be the case? Or maybe you're an identical twin, if so, do you try to trick people for fun? If I were one I'd probably do that. 
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