This is a duplicate of issue636648.

This is intended behavior. It was introduced in changeset 01d5b80a3c03.

From the POSIX Specification [1]:

"""
A pathname consisting of a single <slash> shall resolve to the root directory of the process. A null pathname shall not be successfully resolved. If a pathname begins with two successive <slash> characters, the first component following the leading <slash> characters may be interpreted in an implementation-defined manner, although more than two leading <slash> characters shall be treated as a single <slash> character.
"""

See also Stack Overflow and Stack Exchange questions: [2], [3], [4].

May be we should add an explanation and a link to the specification in the documentation and comments.

[1] http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap04.html#tag_04_12
[2] http://stackoverflow.com/questions/7816818/why-doesnt-os-normapath-collapse-a-leading-double-slash
[3] http://unix.stackexchange.com/questions/12283/unix-difference-between-path-starting-with-and
[4] http://unix.stackexchange.com/questions/1910/how-does-linux-handle-multiple-consecutive-path-separators-home-username