What does // and :: in the values of environment variables mean?

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http://superuser.com – On my Linux machine some of the environment variables contain lists of directories. The directories are separated by :. But sometimes they are separated by ::. Is it just a mistake that should be treated as : or it means that and empty string is one of the directories (that probably should be interpreted as the current directory). Most of the directories specified in the environment variables have this format: /aaa/bbb/ccc However, some of them have this format: /aaa/bbb//ccc Note double slash between bbb and ccc. Is it just a mistake that is interpreted as a single slash or it has a sp (HowTos)