There can be multiple xref types for the same address

[pfalcon]

It's possible to have mov [mem], offset mem. Even more it may come with analyses and transformations on top of raw machine code. For example, I already saw it with "l32r resolution" xform for xtensa.

Ignoring this fact won't work well, at least because there can be competition between different passes what value to store. The way to solve this is to record multiple values - as set, or to save memory, as string of multiple chars.