Find the unpaired message with XOR

A gateway logs every order message twice (send and echo), so each message id should appear exactly twice in the log. One id appears only once, its echo was dropped.

ids = [4, 1, 2, 1, 2]
-> 4

Find the unpaired id in $O(n)$ time and $O(1)$ space. A hash set works but costs $O(n)$ memory, the interviewer wants the bit trick.