beh la specifica URI o URL che dir si voglia prevede esplicitamente il doppio slash e ne spiega le motivazioni
http://www.ietf.org/rfc/rfc2718.txt

2.1.2 Improper use of "//" following "<scheme>:"

Contrary to some examples set in past years, the use of double
slashes as the first component of the <scheme-specific-part> of a URL
is not simply an artistic indicator that what follows is a URL:
Double slashes are used ONLY when the syntax of the URL's <scheme-
specific-part> contains a hierarchical structure as described in RFC
2396. In URLs from such schemes, the use of double slashes indicates
that what follows is the top hierarchical element for a naming
authority. (See section 3 of RFC 2396 for more details.) URL
schemes which do not contain a conformant hierarchical structure in
their <scheme-specific-part> should not use double slashes following
the "<scheme>:" string.


quindi voglio dire... c'è una motivazione tecnica

http://www.ietf.org/rfc/rfc2396.txt
The URI syntax does not require that the scheme-specific-part have
any general structure or set of semantics which is common among all
URI. However, a subset of URI do share a common syntax for
representing hierarchical relationships within the namespace. This
"generic URI" syntax consists of a sequence of four main components:

<scheme>://<authority><path>?<query>

each of which, except <scheme>, may be absent from a particular URI.
For example, some URI schemes do not allow an <authority> component,
and others do not use a <query> component.

absoluteURI = scheme ":" ( hier_part | opaque_part )

URI that are hierarchical in nature use the slash "/" character for
separating hierarchical components. For some file systems, a "/"
character (used to denote the hierarchical structure of a URI) is the
delimiter used to construct a file name hierarchy, and thus the URI
path will look similar to a file pathname. This does NOT imply that
the resource is a file or that the URI maps to an actual filesystem
pathname.

hier_part = ( net_path | abs_path ) [ "?" query ]

eccetera..


per vedere i vari schemi http://www.iana.org/assignments/uri-schemes.html