Prevent a given form submitter from entering the same value in a form field twice (on multiple submits)

functions.php

<?php
function no_duplicate_votes($validation_result){
	global $wpdb;
	$nominee_field = 1;
	$email_field = 2;
	$form = $validation_result['form'];
	
	foreach( $form["fields"] as &$field ) {
		if ( $field['id'] == $email_field ) {
			$email_value = rgpost("input_{$field['id']}");
			$vote_submissions = $wpdb->get_results( $wpdb->prepare( 
				"SELECT lead_id FROM {$wpdb->prefix}rg_lead_detail 
				WHERE form_id = %d and field_number = %d and value = %s 
				ORDER BY id ASC LIMIT 999999", $form['id'], $field['id'], $email_value )
			);

			if ( !empty( $vote_submissions ) ) {
			
				foreach ( $vote_submissions as $vote_submission ){
					
					$last_vote = $wpdb->get_var( $wpdb->prepare(
						"SELECT value FROM {$wpdb->prefix}rg_lead_detail 
						WHERE lead_id = %d and field_number = %d", $vote_submission->lead_id, $nominee_field )
					);
				
					if ( !empty( $last_vote ) ) { 
				
						$nominee_value = $_POST['input_1'];

						if ( $last_vote == $nominee_value ) {
							$validation_result['is_valid'] = false;
							$field['failed_validation'] = true;
							$field['validation_message'] = 'Sorry! You already voted for ' .$nominee_value. ' in this category.';
								continue;
						}
					}		
				}
			}
		}
	}

	$validation_result["form"] = $form;
	return $validation_result;
}
add_filter( 'gform_validation_[form ID]', 'no_duplicate_votes' );