Bit of a strange one this, been talking with @xboxsupport on twitter but not found a solution yet.
I recently removed someone from my friends list who was someone on the list carried over from the 360 days.
Even though they have been removed and do not show up on any of the friends list on Xbox One, Xbox.com and 360 I still get pop up notifications when they come on-line.
I'm running the current build of the Preview update.
anyone else come across this before?