Duplicate filters can sometimes be added via Block element

[Ross]

Environment

ABP 1.11.0.1595
Chrome 32 / 42 / 48
Windows 8

How to reproduce

1. Navigate to the dev build page: ​https://adblockplus.org/en/development-builds
2. Install the latest ABP Chrome build.
3. Open the ABP options page after install.
4. Switch back to the dev build page and refresh it.
5. Select [ABP Icon] > [Block element] and select the ABP icon in the top left corner of the page.
6. Observe two filters are suggested (#logo and .sprite). Add them.
7. Select [ABP Icon] > [Block element] and select anything else on the page.
8. Manually edit the suggested filter to "adblockplus.org##.sprite" and add it.
9. Switch back to the ABP options page > Custom filters.
10. Observe the .sprite filter has been added twice / duplicated.

Observed behaviour

Sometimes, but not all the time (reproduced 3/8 attempts), it's possible to add duplicate custom filters via the Block element functionality. Normally duplicates are ignored, both when adding a filter via Block element and in the options page/tab itself.

Expected behaviour

Attempts to add duplicate filters to be ignored.

[Ross]

I noticed this while testing #3830, although not sure if related.

[sebastian]

Moving to User-Interface, as that is where the responsible message responder is.

[greiner]

Closing this ticket because a lot of the code related to UI mentioned here has changed since. Please create a GitLab issue, if you can still reproduce this issue using the latest builds.