Olly, specifically, will only accept the last prefix it finds if there are more than one from the same group. For example
lock rep add dword ptr[eax],eax ; f0 f3 01 00
??? ; f0 rep add dword ptr[eax],eax ; f3 01 00
Page 28 of the Intel architecture manual simply states "it is only useful to include up to one prefix code from each of the four groups ..." but no where does it say it's required to only have, at most, one from each group. Yes, there are cases where some of the prefixes are required, must not be present, or are only valid depending on the instruction, but I'm not referring to any of those scenarios.
I'm choosing to interpret Intel's guidance as meaning "it's valid but there aren't many reasons to do it" and I'll continue to leave the prefix listed with the instruction (again, barring any specific rules that say otherwise.)
Is this the correct way to handle the situation or should I be doing something different?