diff --git a/installer/data/mysql/atomicupdate/bug_14570.perl b/installer/data/mysql/atomicupdate/bug_14570.perl
index 967fd34a21..c266c9b669 100644
--- a/installer/data/mysql/atomicupdate/bug_14570.perl
+++ b/installer/data/mysql/atomicupdate/bug_14570.perl
@@ -10,6 +10,7 @@ if( CheckVersion( $DBversion ) ) {
                   guarantee_id INT(11) NOT NULL,
                   relationship VARCHAR(100) NOT NULL,
                   PRIMARY KEY (id),
+                  UNIQUE KEY `guarantor_guarantee_idx` ( `guarantor_id`, `guarantee_id` ),
                   CONSTRAINT r_guarantor FOREIGN KEY ( guarantor_id ) REFERENCES borrowers ( borrowernumber ) ON UPDATE CASCADE ON DELETE CASCADE,
                   CONSTRAINT r_guarantee FOREIGN KEY ( guarantee_id ) REFERENCES borrowers ( borrowernumber ) ON UPDATE CASCADE ON DELETE CASCADE
             ) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci;
diff --git a/installer/data/mysql/kohastructure.sql b/installer/data/mysql/kohastructure.sql
index d28822ca0a..cd7d7dfbcc 100644
--- a/installer/data/mysql/kohastructure.sql
+++ b/installer/data/mysql/kohastructure.sql
@@ -4080,6 +4080,7 @@ CREATE TABLE `borrower_relationships` (
       guarantee_id INT(11) NOT NULL,
       relationship VARCHAR(100) NOT NULL,
       PRIMARY KEY (id),
+      UNIQUE KEY `guarantor_guarantee_idx` ( `guarantor_id`, `guarantee_id` ),
       CONSTRAINT r_guarantor FOREIGN KEY ( guarantor_id ) REFERENCES borrowers ( borrowernumber ) ON UPDATE CASCADE ON DELETE CASCADE,
       CONSTRAINT r_guarantee FOREIGN KEY ( guarantee_id ) REFERENCES borrowers ( borrowernumber ) ON UPDATE CASCADE ON DELETE CASCADE
 ) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci;