I would imagine that allowing duplicates risks causing problems. Perhaps there are measures in place that prevent it altogether.

[ What would happen, if he were to try changing his username to "librarian" now, for example? Nothing, an error, a different suggestion? L_ibrarian...

On the topic of librarians, he's curious as to why Angela has brought her book with her — only to have it on hand as reading material, should she grow bored in a city full of blank books, or to annotate it based on her observations? He glances in its direction before moving on to another matter. ]

As we're exchanging contact details, I presume that includes where each of us will be situated for now.