Description:
------------
When script started and do require_once() by relative filename, this function remembers lower case real path.
If script changes working dir via chdir() and directory name consist of not lower-case letters, require_once() will include the file again.

Reproduce code:
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<?php
// This file located under directory with non-lowercase name, i.e. "TesT"
print "<pre>";

require_once("./inc.php"); // single print line there

chdir("./..");
chdir("./TesT");

require_once("./inc.php");

var_dump(get_included_files());
?>

Expected result:
----------------
I expect to single include of inc.php.

Actual result:
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Actually, got two :)