It's pretty simple. The Duplicate Frame Marker is not showing up until you split the clip. Plus, once you've split the clip, it only shows the indicator on the incoming, split clip. I've attached two screen shots to illustrate.
It's a simple edit sequence. No multiclips. Just 3 or 4 video / audio layers.
One note - the markers did show up after closing and re-opening Premiere.
PP V24.1
Mac OS Sonoma 14.2
Studio Ultra
Apple M2 Ultra
GPU
32TB RAID - Thunderbolt
Apple ProRes LT