We can display a stack of warning messages in the "Manage Extension Shortcuts" section of about:addons when a user has installed multiple extensions that try to use the same shortcut. Currently this is implemented as a stack of message-bars via the message-bar-stack element, but now that moz-message-bar is available we should use that instead. While the message-bar-stack component doesn't handle creating message-bars, it's possible the code will need to be modified to work with the newer element.

The HTML code for the stack of duplicate warning messages can be found here, and the JS code that handles adding message bars to the stack can be found here.

https://hg.mozilla.org/mozilla-central/rev/ab4ddf800f7e