Hello Xbox Tenno!
While we were prepping for a special Xbox Anniversary celebration (coming soon) we accidently put live the respective Inbox message containing 400 Ducats (surprise!). Additionally some Tenno received duplicate Inbox messages, bringing their total to 800 Ducats.
We have removed both accidental Inbox messages. If you claimed the 400 or 800 Ducats it's a bank error in your favor.
Apologies for the mix-up. Our Xbox Anniversary celebration will be live soon for all to enjoy!