Steps to reproduce

• Edit an existing ZObject, E.g. https://wikifunctions.beta.wmflabs.org/w/index.php?title=Z10024&action=edit&view=zobject-editor
• It already has a Spanish (Z1003) label
• In the Label block, on the "Select language" lookup field to add a new label, start writing "Spanish"
• GOOD: No option is offered to add "Spanish", as it's already present

Observed behavior

• Directly type Z1003 into the Lookup field
• Select Z1003 (Spanish)
• The existing value for Spanish is removed
• Write a label
• The string is written in both fields
• Observe the JSON representation: one Spanish label is there with the new string and another with the old string

Expected behavior

• No Z1003 Spanish value can be entered
• When the value is selected (with the mouse, or pressing enter or tab) we should check whether the value is duplicated

Additional information

• Here's the demo for this behavior: https://www.loom.com/share/579f783f53ec429f9c05a881eb4d494f
• There are other places where language tags should not be repeated:
  • Label box in zobject editor
  • ZMultilingualString component in zobject editor
  • Argument list in function editor