You could make a script include that's a glidequery based on the field you want to check for dupes in:
function getDupeUsersBy(field) {
return new GlideQuery('sys_user')
.aggregate('count', field)
.groupBy(field)
.where(field, '!=', '')
.having('count', field, '>', 1)
.select()
.toArray(100)
.map(function(u){return u.group[field];})
.join(',');
};
then just set your filter to field is one of javascript:getDupeUsersBy('email');. Replace email with whatever field you want to check, like user_name.
