Check for Duplicate name on registration

Hi,
On the site I have I have members who don't remember stuff, like if they registered before, what the username is or don't have access to their old email ...

So they just re-register with a different username and email. We would like to check if the name they use is a duplicate and let them know that they should contact us for help in retrieving their old account or something.

How would I go about this.

I have found this joomla.stackexchange.com/questions/15344/check-a-form-field-for-duplicates-before-saving
but I don't know where to put it. And I figure it could be done with actions and such

Thank you

Do you want to block the registration if it's a duplicate or just output some kind of warning? If you want to block the registration you can use CB Query Field for that. It provides Query validation that lets you validate a field with a custom database query. You'd just query the name against _users if using single name field or against _comprofiler if using split name fields. Just some kind of warning will be more complicated though as there isn't a built in usage for that and you'd have to implement one.