Repeated #[link] attributes for the same library cause it to be passed as many times to the linker.

> [If I specify #[link(name="pythonXY")] on 30 extern C blocks, and then compile with rustc -l dylib=pythonXY:python27, then python27.lib is passed to the linker 30 times.](https://github.com/rust-lang/rust/issues/37403#issuecomment-267763227)

In extreme cases this may cause trouble on Windows, where the maximum length of the command line is 32000 bytes.  Also, may slow down linking.