Can't recreate form field tag once it's been deleted and the page hasn't been refreshed

[cleydyr]

Steps to reproduce

1. Go to Administration > Catalographic Form Customization
2. Create a new field by clicking on Add Tag
3. Enter 999 on the MARC Tag field
4. Click on the green check mark to add the new field
5. Click on the trash icon corresponding to the 999 field and confirm field deletion
6. Repeat steps 2-4 to add the dummy 999 field again

Expected result

The field will be added again

Actual result

A pop-up will appear saying "Tag already exists"