validate_entry_before_creation

gv-validate_entry_before_creation.php

<?php

add_filter( 'gform_validation', 'validate_entry_before_creation', 100 );
function validate_entry_before_creation( $data ) {
	foreach ( $data['form']['fields'] as $field ) {
		if ( $field->id !== 1 ) { // replace 1 with the unique field ID
			continue;
		}
		$unique_id = RGFormsModel::get_field_value( $field );
		// check if entry exists based on the $unique_id (e.g., passing it to https://docs.gravityforms.com/api-functions/#get-entries)
		if ( /* check failed */ ) {
			$data['is_valid'] = false;
		}
	}
	return $data;
}