Duplicate :focus tags

[jayeff]

Executing the AddFocusTag command on a spec, which has already a :focus tag, duplicates the focus tag.

Example
from it 'does stuff', :focus do to it 'does stuff', :focus, :focus do

AddFocusTag should not duplicate the :focus tag.
Investigate if it makes sense to let the :focus tag bubble up (eg from it to enclosing describe).