It always puzzles me.

For example, given R: There exist x1,x2 ∈X such that x1 ≠x2 and f(x1)=f(x2).

What is not R?

For every x1,x2 ∈X such that x1 ≠x2 and f(x1)≠f(x2) ?

or

For every x1,x2 ∈X such that x1 =x2 and f(x1)≠f(x2) ?

Any suggestion for such kind of problem? (like which part needs to change and which part doesn't)