header("Content-type: image/png"); makes script run twice?

[ZiNgA BuRgA]

I'm guessing you're looking in the wrong direction.
I haven't read everything you posted (including links), but, is your script vulnerable to early termination?  That is, if the user cancels loading the page before the script finishes executing, it's possible for your webserver to kill the script halfway through execution.  I don't know about browsers, but one may guess that it may react differently to various content types sent (browser prefetching or keep-alives perhaps).

You should track what requests the browser is making (use something like Fiddler to log requests) to see if two requests to the script are being made.