Possible Duplicate:
Tag synonym request: [library]->[libraries]
library and libraries seem to be used exactly the same way. Perhaps one of them was once meant to relate to the use of Ubuntu in libraries of the non-computing sense, but if so, then with just one notable exception, that's not how either tag is used in practice.
I'm not sure which tag should be the canonical one, but libraries has a few more questions, so I suggest that library be made the synonym targeting it.
I cannot suggest the synonym here because I don't have a high enough answer score in the tag. Perhaps someone else can suggest it.