mac os x what the two forward slash root is?
This is a strange part of the POSIX specification.

See unix, difference between path starting with '/' and '//'.

Two slashes in the beginning of a pathname are interpreted in an implementation-defined way. As far as I know, only cygwin actually takes // to mean something different than / (//FOO maps to a network drive called FOO, / maps to the local root). On most other systems / and // are the same. Note that /// is interpreted as /.

Your shell is I suspect bash and it keeps the // just in case the OS does something special with it. On OSX it is the same as /.