The only case of double headers I can think of is when using scripts that set a content-type, then try to set it again and the language itself don't prevent it. I think the "right" option in such case would be to follow the last one, as it's the one provided nearer the content. So I vote for using the last one.
PS: Interestingly, this behavior could be used to send xhtml (and get it shown on IE) without sending different headers to each browser: Content-Type: text/html Content-Type: application/xhtml+xml