Removes double occurence of features in feature list.

Before: 

~~~ bash
1354616809.381435 "MONITOR"
1354616840.236863 "get" "feature:tim"
1354616840.237077 "set" "feature:tim" "0|3|"
1354616840.237244 "get" "feature:__features__"
1354616840.237371 "set" "feature:__features__" "tim"
1354616842.463322 "get" "feature:tim"
1354616842.463487 "set" "feature:tim" "0|3|"
1354616842.463613 "get" "feature:__features__"
1354616842.463751 "set" "feature:__features__" "tim,tim"
1354616845.391273 "get" "feature:tim"
1354616845.391451 "set" "feature:tim" "0|3|"
1354616845.391611 "get" "feature:__features__"
1354616845.391768 "set" "feature:__features__" "tim,tim,tim"
1354616853.070983 "get" "feature:ayrton"
1354616853.071137 "set" "feature:ayrton" "0|3|"
1354616853.071331 "get" "feature:__features__"
1354616853.071464 "set" "feature:__features__" "tim,tim,tim,ayrton"
1354616854.999026 "get" "feature:ayrton"
1354616854.999182 "set" "feature:ayrton" "0|3|"
1354616854.999416 "get" "feature:__features__"
1354616854.999546 "set" "feature:__features__" "tim,tim,tim,ayrton,ayrton"
~~~

After:

~~~ bash
1354616809.381435 "MONITOR"
1354616840.236863 "get" "feature:tim"
1354616840.237077 "set" "feature:tim" "0|3|"
1354616840.237244 "get" "feature:__features__"
1354616840.237371 "set" "feature:__features__" "tim"
1354616842.463322 "get" "feature:tim"
1354616842.463487 "set" "feature:tim" "0|3|"
1354616842.463613 "get" "feature:__features__"
1354616842.463751 "set" "feature:__features__" "tim"
~~~