double requests - validation messages not displayed

The problem is, my requests are doubled in production server (apache, shared hosting). The first response is correct, but the second is empty ie. without values passed. As result no validation error messages are displayed. The problem occurs only in production. I am not sure it does matter but I use parser templates for generating response. However I don't modify the response beside setting body to it.
Additionaly I receive the following errors after requests in hosting server error logs (it is probably related):

AH10157: FastCGI: An error happened during Fastcgi processing, fallback to CGI, referer: www.xxx.xx/xx
AH10144: FastCGI: comm with server "xxx xxx" aborted: error parsing headers: duplicate header 'Content-Type', referer: www.xxx.xx/xx

Has anyone any idea, what causes that problem and what could be a solution?