This RT extension adds a "Merge Users" box to the User Administration page, 
which allows you to merge the user you are currently viewing with another
user on your RT instance.  It also adds MergeInto() and Unmerge() functions to 
the RT::User class, which allow you to programmatically accomplish the same
thing from your scrips.  It also provides a version of CanonicalizeEmailAddress,
which means that all e-mail sent from secondary users is displayed as coming
from the primary user.

To install:

    perl Makefile.PL
    make install