The choice of IDs for your users is important; the operating system does not deal with user processes in terms of their user names, it knows only of the numeric identity of a process. In general, it is best if each user has a unique positive user ID (negative IDs are an ugly hack, and proftpd will complain if they are used).
However, in some mass-hosting environments such as ISPs, the number of users is greater than the number of IDs available. In these cases, you can give each user the same ID; additional steps should then be taken to make sure that each user is isolated from affecting other users' files. These additional steps are to make sure each user has a unique home directory, and then to restrict each user to their respective home directories by having
in your proftpd.conf.
There is also currently an issue with using the same usernames in different authentication schemes (e.g. having the same username in both /etc/passwd and a mod_sql SQL table); read this thread for more information: