In the book it states:
I wanted that earring and I wasn't going to just whine about it. When I
get mad, I get determined. When I get mad, I do something. Not always a smart thing.
"You," I said, looking at the starfish. "You could get it back," I said as I took off my outer clothing and stood there in my leotard.
I am assuming that when she took off her outer clothing, she also removed the second earring. Later, psycho!Rachel runs off super quickly, so I am assuming that nice!Rachel was the one who took the outer clothing. Since she found the earring between her toes, she has both earrings. (Or I assume that is what the author intended us to think.)