header goes to new page but leaves orig. page open

[cupboy]

Usually when I use header it just replaces the current page, but now with this code it opens a new tab in firefox and puts in the page there, leaving the original page in the original tab. What can be the cause of that?

code:

if ($_REQUEST['returnitems']) {
header("Location: http://" . getenv(HTTP_HOST) . "/index.php?section=admin&action=returns"); }