What means this error?
command already exists in namespace "::"
Can you provide simple case when this error raises?
That's (probably) an error thrown by the [incr Tcl] core when you try to do something like defining an itcl class with the same name as an existing command that isn't already a class; the part you left out (in quotes, between “command” and “already”) was the name of the command that you were trying to override. For example (the subst is fairly arbitrary here):
% package require itcl
4.0b7
% itcl::class subst {}
command "subst" already exists in namespace "::"
No command that is a “standard” part of Tcl itself (and not an extension package or user code) generates an error that's remotely close.
As for how to fix… don't use the same name as an existing non-class. What this means in the context of your code though, I don't know. You don't reveal enough information for me to work out the deeper reason why you might be having this problem in the first place.