Indicate a duplicate bugwatch after editing it

The fix for bug 29206 (indicate when another bug already uses the same watch) <https://bugs.edge.launchpad.net/malone/+bug/29206> does not notfiy the user about a duplicate when the bugwatch is added or changed during edit operations.

By example:

 1) Go to http://launchpad.dev/bugs/1
 2) Edit one of the bug trackers and change its URL to bugzilla.gnome.org/bugs/show_bug.cgi?id=304070.
 3) Edit another of the bug watches, give it the same URL.
 4) Both bug watches will now have the same bug tracker and remote bug. There should only be one bug watch for each lp bug/remote tracker/remote bug combination.

A better way would be:
1. Use "Also affects project" to add an upstream project to an existing bug.
2. Do not fill in the bugwatch URL, add the project.
3. Edit the project, set the correct bug tracker and bugwatch to a known bug.
4. Verify that a notice is displayed about a duplicate bugwatch.