diff --git a/lib/handlers/on-open.js b/lib/handlers/on-open.js
index 53af3f39..fbeb6091 100644
--- a/lib/handlers/on-open.js
+++ b/lib/handlers/on-open.js
@@ -34,12 +34,13 @@ module.exports = server => (path, session, callback) => {
                 .project({
                     uid: true
                 })
-                .sort([['uid', 1]])
+                //.sort([['uid', 1]])
                 .toArray((err, messages) => {
                     if (err) {
                         return callback(err);
                     }
-                    mailbox.uidList = messages.map(message => message.uid).sort((a, b) => a.uid - b.uid);
+                    // sort and ensure unique UIDs
+                    mailbox.uidList = Array.from(new Set(messages.map(message => message.uid))).sort((a, b) => a - b);
                     callback(null, mailbox);
                 });
         }