In two line notation, this function is \(f = \twoline{1 \amp 2 \amp 3 \amp 4 \amp 5}{1 \amp 1 \amp 2 \amp 2 \amp 3}\text{.}\) From this we can quickly see it is neither injective (for example, 1 is the image of both 1 and 2) nor surjective (for example, 4 is not the image of anything).
in-context