The Latin passive ending usually feature an additional letter R compared to the active endings: laud-or, -aris, -atur, -amur, -antur. However, the second person plural is different, using the ending -amini.
What happened here? How did the ending -mini in the passive evolve?
Bonus question (out of curiosity) Is there an older form of the 2nd person plural passive that contains an additional r?