Can map to same field many times

Description

Steps to reproduce

1. Install an external connector: Campaigns Connector.
2. Configure its settings.
3. Create a form.
4. Map Connect to DataSource = a recipient > Publish.
5. DnD 2 textbox elements.
6. Map the first element to "Email" > Publish.
7. Open the external mapping of the second element.

Expected:
Email is not displayed on the list.

Actual:
Email displays and can be selected.