I don't know which it is, could be wamp too.
Here is what I do. I have two folders in the www folder of wamp. Each has a contact.php file.
I open folder 1, click on a contact.php file, it opens in DW, Preview in browser, opens in IE. I can see the page.
Go to folder 2, click on that file contact.php, opens in DW, Preview in browser, I get a 404 file not found error message.
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