It's possible that the users used a 'clone' function and the chart became duplicated.
It will only be visible to himself and you would not find it when you open you 'master' document.
If this is your case, you can ask the user to remove it ( it would only impact his object) .
It's also possible that some objects are invible, but the visibility condition is met to this user.
You can press Ctrl+Shift+S to see all hidden objects and delete them .
Hope this helps,