Google have an interesting feature in their support forums:
when you post a reference, the system will automatically show which query was used to find it.
Maybe this would be an interesting addition to the dupe-closing process on SO? I.e. when a question gets closed as a dupe, let the first closevoter specify the SO search query they used to find the original - with the default being the very last search query they made. (This would apply to SO searches only!)
It could be a gentle nudge to the OP to check the search next time, educating without being unfriendly.