When I first started studying Latin, I found it mildly curious that the constructions "either...or" (aut...aut), "neither...nor" (nec...nec), and "both...and" (et...et) are formed by repeating the word which corresponds to the latter conjunction in English. It made sense to me, vaguely, but I didn't really think much about it. Many years later, it struck me that the same thing happens with "neither...nor" in Spanish (ni...ni). I wanted to ask my teacher in Mexico whether one translates "either...or" similarly. But she didn't speak English, and I realized how difficult it is to explain what the word "either" really means. In struggling to figure out a definition, I finally realized that these constructions simply emphasize the effect of the conjunction alone; adding "either" to "or" stresses the mutual exclusiveness of two things, just as adding "both" to "and" stresses the inclusiveness. When I thought about it this way, it was completely obvious why the conjunction is just repeated in certain languages.
I never was able to communicate my question to my teacher, but I looked it up later and found that yes, "either...or" is o...o in Spanish. However, "both...and" is translated as tanto...como, which literally means "as much...as," yet another way of expressing the concept.