As ColdFusion is scope based its possible to have a form, request and session variable with the same name. I would advise against this wherever possible as it can make debugging very confusing. Arguments and var scopes can not, however, share the same name. And when they do the (somewhat confusing) error will appear:

Cannot declare local variable userID twice.

Local variables cannot have the same names as parameters or other local variables.

The fix is rather simple simply change the name of the argument or the var!

Here is the code that gives the above error: