HRESULT 0x80072F7B (WININET_E_HEADER_ALREADY

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Description:

The HTTP header already exists

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WININET_E_INVALID_QUERY_REQUEST	WININET_E_REDIRECT_FAILED

WININET_E_HEADER_ALREADY_EXISTS

WININET_E_HEADER_ALREADY_EXISTS is a WinINet HRESULT. It means that the application tried to add a header that WinINet considers already present in the request or header collection. A duplicate can be harmless only when the protocol explicitly allows repeated values; otherwise it can alter request semantics.

What to check

Inspect headers already set by WinINet and the application before adding another value.
Replace, coalesce or intentionally append the header using the documented modifier instead of adding it blindly.
Pay particular attention to headers such as Content-Length, Host, Authorization and Cookie.

Microsoft documentation

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