I did a test, and I was wrong. You can get repeat retinue from the same academy. My games simply go too fast for me to recongize it.

What I don't understand, however, is why it seems that you always get a string of 5-6 (low probaility) retinue right whent he building is produced. I think there's still something fishy going on. Every time I take athens, within 2 turns, I get 5 retinue, generally of the low probability variant (from export_descr_ancillaries.txt). Subsequently, it takes years and years to get even one.

I'm thinking there might be a "burst" at the start that subsequently goes down to normal levels. Basing that only on casual observation, however.