Original Request (from JSIN): 

I may have used the filter assistant to create a filter to send specific emails
to the wrong folder, or have the senders email address in more than one filter.
It would be nice to have a search function to see if you have any duplicates or
a way to trouble shoot any filter issues or track down problems with filters
not working properly.

..

"I think the issue I am having with my filters not working properly is that I
may have several filters containing the same email address or some other field.
This probably happened while creating new folders and trying to better organize
my email."

Requirements:
add a way to find duplicate conditions and actions. (same condition in 2
different filters, same action in 2 different filters)

This will be added in the form of a "find duplicates" button in the filters
list. It will work on each individual mailbox (not across all).

One the button is clicked, a list of matches is displayed in a drop down box.

When an item is selected from the drop down box, the filter list will be
filtered (using the new search feature from Bug 25752) accordingly to show the
matching filters. The filters will get a new context menu item:

  Remove duplicate condition: ....
or 
  Remove duplicate action: ...

detailing what can be changed.

Removal will not happen automatically, instead the Edit Filter window is opened
and the offending condition / action will be scrolled into view and
highlighted. The user can then click the [-] button to remove the condition
himself.

Created an attachment (id=7875) [details]
qF 2.6 prerelease 95

Adding the button (and context menu item) as described.

Note: highlighting actions currently only depends on *type* (not value) so if
you add multiple tags in the same filter the first tag action will always be
highlighted regardless of the value. It is actually difficult to get at the
value.

conditions however should always be highlighted correctly (by value).

Note: the "Find Duplicates" will change to an "x" which will end the search
mode (it currently does not reset the search box contents, but I might change
that). Also, changing to a different mailbox will end the search mode as well.

A detailed description of the feature, complete with screenshots can be found
here:

http://quickfilters.mozdev.org/version.html#2.6

  Firstly, click the new Find duplicate attributes button.This lists filters
that might do the same or determine conditions the same way. 

  Secondly, select a result from the drop down

  This copies the selected condition / action to the search box and narrow down
the list to display the filters that are duplicate candidates

  Then right-click the filter that you want to change (remove the duplicate
condition). This opens up the filter editor, highlighting the duplicate
condition or action in the list

  In a long list of conditions, quickFilters will scroll to the condition and
highlight it with a red background. The user can then delete or edit the
condition.

Created an attachment (id=7876) [details]
Duplicate Search

Created an attachment (id=7880) [details]
qF 2.6 prerelease 105

This version includes a  fix in the loop going through the visible list which
should make it more reliable when the matched duplicate is scrolled off screen. 

It should scroll and highlight. If not, please enable debug mode and post the
log from console2.

(From update of attachment 7875 [details])
made obsolete by prerelease 105