Are there any (sane) guarantees in the UEFI memory map?

I am currently playing around with UEFI/kernel development and am struggling to get a firm grasp on the UEFI memory map returned by GetMemoryMap().
I'd like to know whether there is a guarantee that the memory map does not contain overlaps.
Specifically I seem to be unable to find a statement like "neither the physical nor the virtual address ranges specified in a descriptor may overlap with other physical/virtual ranges (respectively) specified in other descriptors".
More generally I am unable to find just about any guarantees of the memory map layout (sorted would be sane) or references that state the contrary.