Yeah, it happens (if you or I worked in a toy factory in China and had to assemble and paint the same figure, over-and-over again, all day, we'd probably goof up every now and then too). I've got a ML 1st Appearance Spider-man from the Sentinel BAF wave with two identical lower legs. If you've still got your receipt, I'd bring it back and exchange it (assuming they have another one in stock).
I hate it when that happens. It's so irritating because there is no good way to handle it so you're pretty much stuck unless someone happens to have the piece laying around. I used to get this all the time with Black Panther figures.
This would happen once and a while with TB figures, My sentinel has two left biceps and I got a fantastic four movie thing for a custom that had two right thighs. I was pretty frustrated when that happened with the thing because it made it impossible to use it for my custom.