Importer creates duplicate custom fields

Hello, 

we imported today some testcases and we saw that the importer is creating already existing custom fields again. This is messing up our JIRA and the user filters. 

Attached you can find the import logs, an example of a new created custom field and the excel & csv we used to import the testcases. 

The strange thing is that those new created custom fields are not used in the excel file including the test cases. 

Best Regards, 
Philipp