The first preference added to the branch is removed if the second one added was identical at any point

[mythmon]

Transfered from mozilla/normandy#2331 filed by @SoftVision-SimonaRosu

[Prerequisites]:

• Have the Firefox browser installed.
• Have the Normandy Devtools V2.3.1 addon installed.
• Have VPN access to the staging environment.

[Steps to reproduce]:

1. Open the Normandy Devtools and choose the Stage environment.
2. Create a new multi-preference-experiment and populate the fields with valid data.
3. Add a preference to the “Branch” section. (e.g. “browser.newtabpage.activity-stream.showSearch_c”)
4. Add same preference a second time and add extra characters to it (e.g. “browser.newtabpage.activity-stream.showSearch_cbd”)
5. Click the "Save" button and observe the behavior.

[Expected result]:

• Both preferences are correctly added to the branch and the recipe is successfully saved.

[Actual result]:

• Only the second preference added to the branch is saved.

[Notes]:

• This issue is reproducible when creating a new experiment or editing an existing one.
• This issue is not reproducible using the “preference-rollout” recipe type.
• Attached a screen recording of the issue: