The search seems to ignore the new duplicate links when I search for linked questions. (These links have the heading "This question already has an answer here".)
An example for the question "Headers already sent by PHP":
https://stackoverflow.com/search?tab=newest&q=url:%22https://stackoverflow.com/q%2a/8028957/%2a%22%20is: question%2aclosed:1
Notice that newer questions marked as duplicate are absent here, which indicates that the new duplicate links are being ignored. I'm making a request to include them.
Here are duplicates not mentioned in the example above: