using header(), dialog box comes up twice!

[Cailean]

I'm using header() to force a download (or local open) of an xls file. If the user chooses 'save' from the dialog, no problem. If the user chooses open, it asks again, if open chosen again, Excel opens no problem... weird?

My headers are:

header("Content-Type: application/vnd.ms-excel");
header("Content-disposition: attachment; filename=pr_results.xls");

{I promise I've searched and looked through the results...}