I think i may have found it, though I am using Outlook 2007 <> SmarterMail <> TouchDown
I turned off a setting in Outlook to automatically accept meeting requests, some time ago, thinking that would solve it, but didn't.
Today, I poked around in my SmarterMail webmail settings... and found a checkbox under My Settings > Calendar Settings labeled "Add invites as tentative appointments". I have to test it yet, but that was on, so I turned it off. Would make sense that invites from anyone else would end up in my webmail inbox and get approved, and that approval would get sync'd to all my devices unseen (unless you're in the calendar) and then get approved again when I manually did it in outlook later. Meeting UPDATE requests even made the issue worse. Hoping it's fixed... and hope this helps!