Avoid double registrations for same taxonomy type?

Thanks! Sounds both good.
I'm not exactly a drupal ninja when it comes to modules/coding. Any suggestions as to how to see if the user already has the current taxonomy tag in one of his registrations?
If I manage to create the code, i'll post it of course to share..

Another option would be to allow users only to register for one content type. Would that be easier maybe?

Hmm, I'm running into problems (lack of knowledge). Not all has to do with NR though, so I understand if you can't be bothered :D

1. Not sure when to start my module (currently i have: function limit_registrations_by_taxonomy_field_attach_submit($node){ )
In the docs the maker suggests "node_registration_submit". But AFAIK this is not a hook, so how to use that?
2. I managed to get all nodes with a taxonomy term, but can't figure out how to get the page's current term.
3. Not sure how to fetch all node registrations for a given userid. Should I use _node_registration_user_registered($node, $account) ?
4. How to make node_registrations stop registering a user, when my code indicates that yes indeed this user was already registrered for this term..

Programming is not my thing yét ;)

right now, I have difficulty using
hook_node_registration_access($registration, $op, $account, $reason) {

The description of the parameters is not completely clear to me:
$registration - it is a new registration, so no id yet. So what do I pass to the function?

so far I've got something like:

function limit_registrations_by_taxonomy_node_registration_access(null,'add',$account->uid;) {
 if($alreadyregistrered){
return FALSE;
}
}

Solved it! :)

function limit_registrations_by_taxonomy_node_registration_access(){      
    //load current node
    $node = menu_get_object('node');
    //load current taxonomy term
    $currentTaxId = $node->field_type_clinic['und']['0']['tid'];
    
    //get all the nodeid's that contain the taxonomy terms from the current page
    $nodesToCheck = taxonomy_select_nodes($currentTaxId,FALSE,FALSE);
        
    //loop over the taxonomy id's and check for registrations. Return FALSE if any registration is found.
    foreach ($nodesToCheck as &$value) {
        if(_node_registration_user_registered(node_load($value))){
            dpm("al ingeschreven voor ".$value."!");
            return FALSE;
        }
    } 
}

Only thing left: how to set the hook propperly, so that a message instead of a ? is displayed to the user.
function hook_node_registration_access($registration, $op, $account, $reason)
I don't know what to set for $registration, $op, $account, as it all seems to work without providing those? I want however to set $reason..

Final:
not solved:

• message to explain the user why he can't register, instead of the questionmark

prerequisites:

• you need a taxonomy field on the workshops page and use the right machine name. In this case it's called field_type_clinic

function limit_registrations_by_taxonomy_node_registration_access(){
    //load current node
    $node = menu_get_object('node');
    
    
    if(isset($node->field_type_clinic['und']['0']['tid'])){
        
        //load current taxonomy term
        $currentTaxId = $node->field_type_clinic['und']['0']['tid'];
        
        //get all the nodeid's that contain the taxonomy terms from the current page
        $nodesToCheck = taxonomy_select_nodes($currentTaxId,FALSE,FALSE);
            
        //loop over the taxonomy id's and check for registrations. Return FALSE if any registration is found.
        foreach ($nodesToCheck as &$value) {
            if(_node_registration_user_registered(node_load($value))){
                //dpm("already subscribed for ".$value."!");
                //user should not be able to register
                return FALSE;
                //return $reason = "You can only register for one workshop.";
            }
        }
    }
}