whats the meaning of ::
colon in PATH environment?
/usr/local/bin:/usr/bin:/usr/local/gnu/bin::.:/usr/local/bin:/usr/bin:/usr/local/gnu/bin:/usr/local/bin
and whats the meaning of // in PATH ?
It doesn't mean anything. It shouldn't be there. The : is for seperating directory names. If :: is there then there is a typo. Also, I would not recommend a . (dot) in any PATH setting.
It looks like someone tried to modify the PATH and screwed it up.
export PATH="/usr/local/bin:/usr/bin:/usr/local/gnu/bin"
I think this is what you need.
-X