It's not that unusual. I think it's rare that someone would specifically wear 2 t-shirts, but it's extremely common for people to wear a t-shirt over a long-sleeved shirt, or for women to wear a long tank top underneath a shorter tank top (or variations on this theme) for a layering effect, at least in America.

I would say two t-shirts is still unusual. To me it would be almost as strange as wearing two button-downs at once. Layering is pretty much always done with two items of clothing that are different, i.e. a t-shirt paired with a long sleeve shirt / sweater / jacket. If you want the t-shirt visible, you could wear a wifebeater underneath or an open-faced long sleeve shirt on top.

I'd disagree with the other commenter that a long sleeve underneath a t-shirt is a good look. I've seen people wear it, but this look is mostly something that I'd expect a middle schooler to wear.