I only see her once, unless you thought Ookouchi Shino was her. But in any case, when they have them twice, there's some difference, like Azusa is on there twice, but one is the tan one.
Gimeul said: The difference is in the eyes. The first Nagi has shine in her eyes, while the second one doesn't... Kinda like her eyes are, well, empty.
I never read the manga or watched the anime, so I'm not sure if there's a reason for this.
Sort of a split-personality deal, not sure if she's aware of it. You could think of it as "brat mode" and "goddess mode."
The doubled characters: Haruhi (Younger Ver.) Nagato (Alt Universe Ver), Mikuru (Future Ver.), Ritsu (Headbandless Ver.), Azusa (Tanned Ver.), Nagi (Goddess Mode), Tsugumi (Waitress Ver.), Churuya, Kurokona, Kyonko, Itsuki (Female), Kagamiku, and if you want to get technical, Hakua
I think that's all of them. Essentially, they're alternate versions of character in fighting games. Note that they're placed after all the other characters in their series and right before the next series.
Shana + Aisaka Taiga + Minase Iori = Kugimiya Rie Team Suzumiya Haruhi + Izumi Konata + Chibi Haruhi + Kurokona (any three of the four members) = Hirano Aya Team