I recently tried to run two identical ASUS ROG Strix XG27UCS 4K monitors through a CalDigit Element 5 hub on my M4 MacBook Pro. Instead of plug-and-play dual extended displays, only one monitor would wake up at a time.
After running through standard isolation tests—dropping refresh rates to rule out Thunderbolt bandwidth limits, bypassing the hub entirely, and testing the monitors individually—it became clear the hardware was fine. The Mac simply refused to initialize a second display stream when both were connected.
The Diagnosis
I looked at the display details in BetterDisplay to pull the parsed data.
Sure enough, the vendor ID, product ID, year and week of manufacture, and internal serial numbers for both monitors were 100% identical byte-for-byte.
As Alin Panaitiu notes in a write-up on weird monitor bugs:
The problem comes from vendors who flash the same exact firmware with the same EDID to multiple monitors in the same batch.
Serial number is emphasized above especially because by definition it should be a unique number that increments with each monitor. Well it doesn’t. And it has nothing to do with the serial number you see on the back of the monitor or the box or wherever it’s placed.
So if you have the bad luck to buy two identical monitors, from the same batch, with the same EDID, then the OS will compute the same UUID for both.
I apparently had exactly the "bad luck" Alin is referring to, despite purchasing my two monitors several months apart. They both had a "serial number" of 1010101.
The Software Workaround
While the bulletproof fix is buying a hardware EDID emulator dongle (and the DisplayPort adapters & cables to go with it), I opted to solve it with software.
I exported the monitor's profile, used an EDID editor to modify the serial string, recalculated the checksum byte, and saved a new .bin profile. Using BetterDisplay, I injected this custom EDID override at the OS level.
By spoofing an EDID for the second monitor, the Mac finally saw two distinct devices and lit them both up. It’s an imperfect hack—it creates a slight race condition if you change display resolutions—but it gets the job done without having to order dongles and adapters and waiting on shipping.
Closing Thought: Missing Windows
This was frustrating enough to actually make me miss Windows (and Linux). Both handle identical monitors gracefully by tracking the physical hardware connection path (port topology) rather than treating the EDID as the absolute, infallible source of truth.
And it's not just the display topology. This whole fiasco also reminded me of Windows' superior window management. Having straightforward, native keyboard shortcuts to snap windows into quadrants or throw them across different displays is something macOS still frustratingly lacks out of the box. Sometimes, the Apple ecosystem requires a lot of under-the-hood work to actually "just work."