I'm trying to figure out how the IBM 5150 PC handled the case where multiple physical devices (memory chips) were mapped to the same address within the 8088's physical address space.
The closest I've been able to find is in the August 1981 IBM 5150 Technical Reference, where it says on page 2-68 (PDF page 87) about input from I/O address 3BCH or 378H that
This command presents the CPU with data present on the pins associated with the out to x'3BC'. This should normally reflect the exact value that was last written to x'3BC'. If an external device should be driving data on these pins (in violation of usage ground rules) at the time of an input, this data will be 'or' ed with the latch contents.
Though of course, not only is that about port I/O, not memory-mapped devices, but also about the MDA/printer combo card, so it's not necessarily the same thing at all (though it could be).
Minus Zero Degrees cautions that the base address for extension memory adapters need to be different from those of any other extension memory adapter in the system. For example, about the 64 KiB adapter:
The switches control the starting address of the RAM. /.../
You can have multiple of these cards in your IBM 5150 or 5160, providing that none of the RAM on a card has an address conflict with any other RAM (motherboard RAM or expansion card RAM).
In order to have a reliable system and get the full benefit of the extra RAM installed, this makes perfect sense: No memory address conflicts allowed.
But what actually happened if you did have a memory address conflict?
Did the system refuse to boot (POST failure)? Did it boot, but functioned erratically depending on which card just happened to serve the particular memory I/O request each time? Or did it boot and functioned "just fine", except you didn't get the benefit of all the extra RAM you just installed (and if so, what about additional ROM chips)? Or what?
Bonus points for answers that not only discuss the user-visible behavior, but also what was going on electrically and/or logically inside the computer.