diff --git a/src/app/Backends/LDAP.php b/src/app/Backends/LDAP.php
--- a/src/app/Backends/LDAP.php
+++ b/src/app/Backends/LDAP.php
@@ -713,6 +713,13 @@
         $domainBaseDN = "ou={$domain->namespace},{$hostedRootDN}";
 
         foreach ($group->members as $member) {
+            $member = strtolower($member);
+
+            // filter out duplicate members
+            if (in_array($member, $validMembers)) {
+                continue;
+            }
+
             list($local, $domainName) = explode('@', $member);
 
             $memberDN = "uid={$member},ou=People,{$domainBaseDN}";
@@ -747,7 +754,10 @@
 
         // Update members in sql (some might have been removed),
         // skip model events to not invoke another update job
+        sort($validMembers);
+
         $group->members = $validMembers;
+
         Group::withoutEvents(function () use ($group) {
             $group->save();
         });
diff --git a/src/app/Group.php b/src/app/Group.php
--- a/src/app/Group.php
+++ b/src/app/Group.php
@@ -197,7 +197,10 @@
      */
     public function setMembersAttribute(array $members): void
     {
-        $members = array_filter(array_map('strtolower', $members));
+        $members = array_unique(array_filter(array_map('strtolower', $members)));
+
+        sort($members);
+
         $this->attributes['members'] = implode(',', $members);
     }