1. Create new content type with three the same field types, for ex. textline. Leave default identifiers.
2. Save
3. Edit your content type
4. Delete the first and second field type
5. Add two more the same field types - error: it's impossible to add the third field type: "Argument '$fieldDefinitionCreateStruct' is invalid: Another FieldDefinition with identifier 'new_ezstring_3' exists in the ContentType"