According to the POSIX specification, paths that begin with exactly two slashes have implementation-defined semantics.
Multiple slashes in a path are collapsed to one slash, except for the case where there is exactly two slashes at exactly the beginning of the path.
So, /foo, ///foo, ////foo, and ///////////////////foo are guaranteed to be the same path, /foo/bar, /foo//bar, /foo///bar, and so on are guaranteed to be the same path, but /foo and //foo are not guaranteed to be the same path, and neither are //foo and ///foo – any implementation can define the semantics as they wish. They could choose to define //foo to mean the same thing as /foo and ///foo, but they don't have to.
The intention is that operating systems can use paths beginning with // to implement semantics that are different from POSIX file system semantics.
For example, a hypothetical hybrid of Windows and Unix could use // for Windows filesystem semantics. Cygwin actually uses // for network paths, similar to how Windows uses \\.
In Cygwin, for example, cd //; ls would list all SMB fileserver on the local network, not the root directory!