Opera browser has this feature,
it underlines names in tabs with blue, if there are multiple instances open from same URL.
Also a right click option to remove duplicates.
Thanks for submitting an idea to the Mozilla Connect community! Your idea is now open to votes (aka kudos) and comments.
Similar idea here: Suggestion: Add "Close Duplicate Open Tabs" option to browser tab right click option
Visual feedback for duplicate tabs
Please implement visual feedback in tab bar for duplicate tabs. For example, in Opera if hovering over tab and if that tab has duplicate, both of them are highlighted.
(Note: similar ideas have been merged into this thread)
never-displayed