diff --git a/CHANGELOG.md b/CHANGELOG.md
index a61d7d5f11a..91c8827a7e1 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -26,6 +26,7 @@
 - Password: Allow LDAP access using LDAP URI and SASL binding (#8402)
 - Fix potential HTTP protocol version mismatch (#8982)
 - Fix PHP8 fatal error when parsing a malformed BODYSTRUCTURE (#9171)
+- Fix duplicated Inbox folder on IMAP servers that do not use Inbox folder with all capital letters (#9166)
 
 ## Release 1.6.4
 
diff --git a/program/lib/Roundcube/rcube_imap_generic.php b/program/lib/Roundcube/rcube_imap_generic.php
index 8334a7f93b4..fe36d489269 100644
--- a/program/lib/Roundcube/rcube_imap_generic.php
+++ b/program/lib/Roundcube/rcube_imap_generic.php
@@ -1658,6 +1658,18 @@ protected function _listMailboxes($ref, $mailbox, $subscribed = false, $return_o
                         $mailbox = rtrim($mailbox, $delim);
                     }
 
+                    // Make it easier for the client to deal with INBOX folder
+                    // by always returning the word with all capital letters
+                    if (strlen($mailbox) == 5
+                        && ($mailbox[0] == 'i' || $mailbox[0] == 'I')
+                        && ($mailbox[1] == 'n' || $mailbox[1] == 'N')
+                        && ($mailbox[2] == 'b' || $mailbox[2] == 'B')
+                        && ($mailbox[3] == 'o' || $mailbox[3] == 'O')
+                        && ($mailbox[4] == 'x' || $mailbox[4] == 'X')
+                    ) {
+                        $mailbox = 'INBOX';
+                    }
+
                     // Add to result array
                     if (!$lstatus) {
                         $folders[] = $mailbox;