One-to-One function? In text it states that a one to one function is “A function f is one to one if for any two range values f(u) and f(v), f(u)=f(v) implies that u=v. What exactly does this mean? I thought if there are two equal y values it is NOT a one to one function?

Image of two different elements cannot be same. Consider $f:\mathbb R \to \mathbb R, f(x)=x^2$. Here $f(u)=1$ for what values of $u$?
