Using a Gravity Forms filter to create a custom validation and check for existing UID in already saved entries.

gravity-forms-validation.php

<?php 

/***************************************************************
* Function media_custom_validation
* Check the media form for duplicate inputs on the same day
***************************************************************/

add_filter('gform_validation', 'media_custom_validation');

function media_custom_validation($validation_result) {
 	
 	global $form_email, $file_id;

	// new gravity form 
	$rg = new RGFormsModel(); 
		
 	// grab the form   
        $form = $validation_result["form"];
 
 	// only validate the vault form
  	if ($form['title'] == "Vault") {
		
		foreach($form['fields'] as &$field) {

			// store email field in variable
			if ($field['id'] == "2") {
				$form_email = rgpost("input_{$field['id']}");
			}
			
			// store file ID in variable
			if ($field['id'] == "3") {
				$file_id = rgpost("input_{$field['id']}");
			}
			
			// check the uid field
			if ($field['label'] == "UID") {
				
				$value = rgpost("input_{$field['id']}");
				$uid = md5($value.$form_email.$file_id);
				
				// search gravity forms for this UID
				$search = $rg->get_leads($form['id'], 0, 'DESC', $uid, 0, 1000);				
				
				// if UID already exists
				if (count($search) > 0) {  	

		     	 	// set the form validation to false
		    		$validation_result["is_valid"] = false;
		    	
			    	// specify the first field & second field to be invalid and provide custom validation message
			        $form["fields"][0]["failed_validation"] = true;
			    	$form["fields"][0]["validation_message"] = __('You have already downloaded this file today.', 'sandb');
			   	$form["fields"][1]["failed_validation"] = true;
			    	$form["fields"][1]["validation_message"] = __('You have already downloaded this file today.', 'sandb');
			    				    
			    	// update the form in the validation result with the form object you modified
			    	$validation_result["form"] = $form;
			    }
			}
		}	
	}
    return $validation_result;
}