@is_12 

Try below approach.

1. Write an onChange client script on primary contact as below"
   

   function onChange(control, oldValue, newValue, isLoading) {
       if (isLoading || newValue == '') {
           return;
       }
   
       // Call GlideAjax to check existing RITMs
       var ga = new GlideAjax('CheckUserRITMs');
       ga.addParam('sysparm_name', 'hasExistingRITM');
       ga.addParam('sysparm_user_id', newValue);
       ga.addParam('sysparm_catalog_item', g_form.getUniqueValue()); // catalog item sys_id
       ga.getXMLAnswer(function(response) {
           if (response == 'true') {
               g_form.showFieldMsg('primary_contact', 'This user has already requested this item.', 'error'); //check field name
           } else {
               g_form.hideFieldMsg('primary_contact', true); //check field name
           }
       });
   }
   ​

2. Write a New Client callable script include as below
   Name: CheckUserRITMs
   Accessible from: All application scopes
   Client Callable: True
   

   var CheckUserRITMs = Class.create();
   CheckUserRITMs.prototype = Object.extendsObject(AbstractAjaxProcessor, {
       
       hasExistingRITM: function() {
           var userId = this.getParameter('sysparm_user_id');
           var catalogItemId = this.getParameter('sysparm_catalog_item');
   
           if (!userId || !catalogItemId) {
               return 'false';
           }
   
           var ritmGR = new GlideRecord('sc_req_item');
           ritmGR.addQuery('cat_item', catalogItemId);
           ritmGR.addQuery('u_primary_contact', userId); // replace with actual field name if different
           ritmGR.query();
   
           if (ritmGR.hasNext()) {
               return 'true';
           }
   
           return 'false';
       }
   
   });
   

This will show an error message at the field level and also user cannot submit even if they click the order button.

Accept the solution and mark as helpful if it does, to benefit future readers.
Regards,
Sumanth