Question for the guys running a DD fixed front sight and flip up rear. What rear BUIS are you using in conjunction with the DD front? It seems like not all Irons are the same heights so I'm curious, Troy rear?

The only rear sights I know of that won't work are the actual micro sights.  All others should be a standard height.  When you see sight height differences, it usually the front sight that is different.  Some are designed to mount on a "standard" railed gas block, which is lower than the receiver rail.  Others are designed to mount onto a railed hand guard, or a taller railed gas block, both of which are referred to as same plane height.  They are at the same height as the upper receiver rail.

This.  FWIW, I use a MBUS Pro Rear on my rifle w/ a DD fixed Front.

Thanks guys, I'm running a fixed set of DD irons on a M4 flat top and free float rail. have been thinking of running a folding rear with fixed DD front. and using the Dot and front more but with lower 3rd co witness the the irons aren't as prominent as would be with absolute co witness.
I had a DD front and Troy rear that were extras and looking at the 2 next to each other they were different heights so it made me wonder.

OP references a fixed front sight.  Any standard height fixed or folding rear sight will work with it.  If you want to use an optic, especially a scope, a folding rear would be needed.  The inexpensive polymer Magpul MBUS Gen 2 works fine.  The metal MBUS Pro is slimmer and folds flatter.  The Pro is my current favorite.

I used a Troy rear with a DD fixed front for a long time.  Works perfect.

KAC 3-6 Micro with my DD front. I have also used a Matech with it without issue