I can use planning and cardwall without tons of warnings

Note: by no mean this acknowledge that children with multiple parent are supported. It's just a cosmetic change to make things a little bit more usable

• On planning v1, Content and cardwall
  • The feedback box for all but errors is fixed size and display only the 5 first message with a "..." button
  • When one click on the button, the whole list of messages are displayed
  • After having displayed everything, the user can collapse the feedback again
• On planning v2
  • When a card is about to be loaded and displayed twice (ie. a task children of 2 stories):
    • both cards are identified with a warning icon and a popup that explains the issue
    • a message is collected at the top of the page (equivalent of the feedback in planning v1)