I also had this issue and on the advice of another forum member I disabled C-State control in the BIOS today (this controls the processor going into various power saving states when it is idle). This is different to speed step which alters the speed when the processor is in use. Seems to have done the trick for me but I suggest you do your own research.
From my limitied understanding it will likely use more power and may increase temperature a bit, but so far all good for me and no keyboard repeats.
Obviously it would be better if Dell or their partners could fix whatever is causing the issue in the first place.
Mine is doing the exact same thing (external keyboard)-- have tried multiple keyboards, with or without usb hubs, updated all the software, including firmware, erased windows and reinstalled, Dell has even replaced the motherboard yesterday and same issue.
My issue really comes out with an external monitor and the dell xps lid closed. Keyvboooooooooooooooooo) will do that -- and may or may not even detttttttttttttttttteeeeeeeeeeeeeeeeeect an input