SN22.58: http://www.accesstoinsight.org/tipitaka ... .than.html. Other differences are described between these three types than that the Arahant succeeds in the presence of an already existing doctrine, the solitary Buddha doesn't generate an era in which the dhamma is taught, and the Samma Sambuddha does. The Buddha has for example the "ten powers of the Tathagatha".retrofuturist wrote: Somewhere I'm pretty sure there's a sutta where the Buddha explains how he is different from the Arahants and the main reason is that he discovered the Dhamma himself... but I can't seem to find it anywhere just at the moment.
It's never been clear to me, though, that there was any need for a further explanation of why there aren't two at a time, than to say that as a matter of definition it's not until the teachings are no longer current that a new one can arise. This leaves me a little puzzled that the Milindapanha in particular offers further reasons, that it would be like having twice as much food at a meal and so on, as if it were a problem one could imagine arising. I would think that were a second individual with similarly great paramis to be born at about the same time, it would simply be an extra bonus for the rest of the people living at the time. Whoever it is who was first to reach the deathless would be the Buddha, and the other could "just" be a chief disciple with a lot of ability. These types of individuals are all rare enough to begin with that it hardly seems likely that there'd be a problem of oversupply.
So I guess what puzzles me is not why there is only one "Buddha" at a time, but why it was considered useful to say so, and to give more than what seems the most obvious reason for it.