I don't know what happened but somehow it seems like the upload instance (the flash button and the Queue Items) got duplicated on page load. I've tried to use the same settings as i did on my page that bugged it self on another page and it worked on the other one.
This was not an problem before but after some time when i did continue my work in general i came back and saw that this duplicate error showed up. I've checked after some sort of duplicate .uploadify functions but there is none.
Someone that could give me some form of hint on where the problem might be.
Fixed it. The problem was related to the javascript API Highcharts. The way to bypass it is to simply not include it in the same page. Otherwise i don't know how to go around it.