BRIEFEST BRIE FEST, what an evocative phrase — at least to this lactose-intolerant imagineer. Oh yes, thank you, I'll take a chunk of that BRIE … uh oh … (groan) ... gotta run!

Doesn't stop me from eating cheese, sadly enough for the members of my quarantine household.
I enjoyed the non-duplicative duplications. Such fantastic consistency in the pattern: every one starts with an eight-letter word, then follows with that word broken into four and four. Perfect!
Two of them worked great: BRIEFEST BRIE FEST and FLAGRANT FLAG RANT. Took me a while to figure out why the others fell flat. While HEATHENS HEAT HENS amused me, in a headline sort of manner, that style didn't mesh with the two winners.
And MUSTACHE MUST ACHE? It's a neat discovery. It's not a neat phrase to say. Reminds me of the ecstatic pain that comes with compulsively plucking my facial hair (much to Jill's chagrin).
What other themers would be possible? Ooh, an algorithmic problem to solve! It's easy to generate a list of eight-letter entries that can split 4/4 into two valid words. There would be a ton of false positives, though, since you'll also catch multi-word entries like LESS THAN, FOUR LEAF, HALF MOON, etc.
I do keep a separate list of entries with spaces intact, so I formulated a way of exploring that search space while simultaneously cross-checking—
Huh? Right, you stopped listening three paragraphs ago.
(My favorite find was PLUMPEST PLUM PEST.)
Fun concept, and great gridwork to match—Natan, Andy, and the JASA class did a fantastic job keeping out gluey bits while maximizing their mid-length slots with snazzy JAYBIRD, DUE DATE, THE ARMY, AS USUAL. Such strategic black square placement to separate themers, and excellent execution in the tough west and east sections, where many constructors typically falter in a layout like this.
If all four themers had followed that fun (adjective) + (adjective) + (noun) pattern, I'd have given it POW! consideration. Slimming down to just three fantastic themers could have done it, too, especially since that would have allowed for even more goodies in the fill.