Revert "Make index not unique (#537)"

This reverts commit ea2b38f.

I decided that since the code is written
with the assumption that there is only one
issue per person that I would revert this.
If the logic changes I will change this to
non-unique.

db/install.xml

@@ -56,7 +56,7 @@
         <KEY NAME="customcert" TYPE="foreign" FIELDS="customcertid" REFTABLE="customcert" REFFIELDS="id"/>
       </KEYS>
       <INDEXES>
-        <INDEX NAME="userid-customcertid" UNIQUE="false" FIELDS="userid, customcertid"/>
+        <INDEX NAME="userid-customcertid" UNIQUE="true" FIELDS="userid, customcertid"/>
       </INDEXES>
     </TABLE>
     <TABLE NAME="customcert_pages" COMMENT="Stores each page of a custom cert">

db/upgrade.php

@@ -203,7 +203,7 @@ function xmldb_customcert_upgrade($oldversion) {
     if ($oldversion < 2023042403) {
         // Define index to be added to customcert_issues.
         $table = new xmldb_table('customcert_issues');
-        $index = new xmldb_index('userid-customcertid', XMLDB_INDEX_NOTUNIQUE, ['userid', 'customcertid']);
+        $index = new xmldb_index('userid-customcertid', XMLDB_INDEX_UNIQUE, ['userid', 'customcertid']);
 
         // Conditionally launch add index.
         if (!$dbman->index_exists($table, $index)) {