Infinite loop when enqueuing CDN with duplicate entries

Problem/Motivation

I just stumbled upon an issue with the CDN submodule, where when I have the same URL in the config form added twice, then the enqueuing process comes into a loop, in which hundreds of thousands of queue items are generated in the queue for the same URL until the PHP process times out.

Steps to reproduce

• Go to Settings (admin/config/development/warmer/settings).
• Here in the CDN section under URLs add the same URL twice and save the form.
• Go to Warm Caches (admin/config/development/warmer), select CDN from the list and then submit the form by clicking on Warm Caches.
• Result: Form submission takes forever, in which time the queue gets filled with hundreds of thousands of queue items for the same URL entry.

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes