1edo
One note repeated in octaves is an example of a trivial temperament. That said, it is actually useful in some cases from a musical standpoint- particularly as a subset of larger EDOs.
The first piece of Musica ricercata by György Ligeti simulates 1edo by playing only one note throughout different octaves on the piano, though the piano is not itself tuned to 1edo. The last note of the piece breaks the pattern.