I'm not sure if this is a navigation issue. Two dupes have oxygen at 0.00 but do not die, they just keep jumping back and forth. They also should have a clear path up the ladder above them. Could not duplicate issue using earlier save file.
Note: They did suffocate upon reloading the save file, supernova.sav, but then they had "Injuries: Paralyzing"
I was creating an inverted triangle of flooring, dupes may have gotten on the same navigation path.