Skype got message leak bug.

Skype has a new message leak bug. That means your messages to one of your contact may get automatically forwarded to another user in your contact.

Skype Message Leak

Thankfully Skype officials are aware of the bug and is trying to fix it. This bug was found by one of the members in Skype support forum.

So you better hold on to sending those private messages before Skype fixes the bug.