The same recipe slug can wrongly be set for multiple multi-preference-experiment and branched-addon-study actions

[Softvision-PatriciuPop]

[Affected Platforms]:

• All Windows
• All Mac
• All Linux

[Affected Versions]:

• Delivery Console v1.127.0

[Prerequisites]:

• Have access to the Stage Delivery Console.
• Have a multi-preference-experiment already created.

[Steps to reproduce]:

1. Open the browser and navigate to "https://delivery-console.stage.mozaws.net/".
2. Create a new multi-preference-experiment and populate the fields with valid data.
3. Add the same value in the "Experiment Name" as the the multi-preference-experiment from prerequisites.
4. Click the "Save" button and observe the behavior.

[Expected result]:

• The "Branch Slug" fields are highlighted and an error message is displayed.

[Actual result]:

• The recipe is saved.

[Notes]:

• When two branched-addon-studies with the same slug are executed, the client enrolls in both experiments.
• When two multiple multi-preference-experiments with the same slug are executed, the client is only enrolled in one of them.
• Attached a screen recording of the issue:

[mythmon]

Although it would be nice for DC to reject this before submitting it to the server, it is required that the server should reject bad data independent of any UI. This is really a problem with the normandy server, not with DC.