ERR_NGROK_375: Duplicate 'add' header, HEADER was provided twice with different casings: CASE_ONE and CASE_TWO - ngrok documentation

What does ERR_NGROK_375 mean?

ngrok returns ERR_NGROK_375 with this message:

Duplicate 'add' header, <HEADER> was provided twice with different casings: <CASE_ONE> and <CASE_TWO>.

Where can I get more help with ERR_NGROK_375?

Email support@ngrok.com and include the error code.