Duplicate impl blocks in rustdoc

[caipre]

The inherent methods for structs in std::os::unix::net are shown twice, eg: http://doc.rust-lang.org/nightly/std/os/unix/net/struct.SocketAddr.html

Not sure whether this is limited to nightly or not.

Possibly related to #21474.

[GuillaumeGomez]

It's "supposed" to have been fixed in this commit.

cc @alexcrichton