What does double slash // in `cd //` mean in Linux? [duplicate]

Actually it means nothing and is ignored.

From the Bash FAQ E10::

E10) Why does 'cd //' leave $PWD as '//'?

POSIX.2, in its description of 'cd', says that three or more leading slashes may be replaced with a single slash when canonicalizing the current working directory.

This is, I presume, for historical compatibility. Certain versions of Unix, and early network file systems, used paths of the form //hostname/path to access 'path' on server 'hostname'.

Also the Unix standards states:

A pathname that begins with two successive slashes may be interpreted in an implementation-defined manner, although more than two leading slashes shall be treated as a single slash.

This answer on AskUbuntu confirms that they are the same directory. Further, it helpfully quotes from the POSIX Pathname Resolution specification:

A pathname consisting of a single slash shall resolve to the root directory of the process. A null pathname shall not be successfully resolved. A pathname that begins with two successive slashes may be interpreted in an implementation-defined manner, although more than two leading slashes shall be treated as a single slash.