Enrolment

Before doing any of this, please do a backup of your Moodle database,
just in case :-)

Next, make a backup copy of the ../moodle/admin/uploadusers.php
file. Then edit that file and look around line 250 (line number may
vary among different 1.6.x versions) for a piece of code that looks
like this:

      } else {
          //Record not added - user is already registered
          //In this case, output userid from previous registration
          //This can be used to obtain a list of userids for existing users
          notify("$user->id ".get_string('usernotaddedregistered', 'error', $username));
          $userserrors++;
          continue;
      }

and remove the line that reads 'continue;'.

That should do it. Remember you'll have to do this each time you
upgrade your Moodle installation.

Saludos. Iñaki.