Fix duplicate warnings with `raw` calls

There was a bit of a logic error here, because a
warning might be detected as a duplicate in
`check_for_immediate_xss`, causing it to return 'false'
and then the check would go ahead and process the `raw` call.
This would result in 'weak' confidence warnings that were
duplicates of 'high' confidence warnings.

lib/brakeman/checks/check_cross_site_scripting.rb

@@ -137,7 +137,7 @@ class Brakeman::CheckCrossSiteScripting < Brakeman::BaseCheck
   #Otherwise, ignore
   def process_escaped_output exp
     unless check_for_immediate_xss exp
-      if raw_call? exp
+      if raw_call? exp and not duplicate? exp
         process exp.value.first_arg
       end
     end