[jQuery] Validation Error message help

[Dave Maharaj :: WidePixels.com]

I have this script:
 
$(document).ready( function() {
 
    $('#username').blur( function () {
 
        fieldName = $(this).attr('id');
        fieldValue = $(this).val();
 
        $.post('/users/ajax_validate', {
                                        field: fieldName,
                                        value: fieldValue
                                        },
               function(error) {
 
                   if(error.length != 0) {                
 
                       $('#username').after('<div class="error-message"
id="'+ fieldName +'-exists">' + error + '</div>');
                   }
                   else {
                       $('#' + fieldName + '-exists').remove();
                   }
               });
     });   
 
});
 
So when the user leaves the field it checks validation. If error it shows an
error message. Everything is working fine except if the user goes back to
the field where there was an error and does not chnge anything and leaves
the field again the error message is duplicated.
 
So they enter a username peter it says "Please choose a different name" if
they go back to the username and thenleavewith out changing "peter" i now
have:
 "Please choose a different name"
"Please choose a different name"
 
What do I have to change / add so that only 1 message will appear?
 
Thanks
 
Dave