[Resolver] Fix performance issue with exponential growth of dependency nodes cache

CHANGELOG.md

@@ -8,7 +8,8 @@ To install release candidates run `[sudo] gem install cocoapods --pre`
 
 ##### Enhancements
 
-* None.  
+* Improve performance of the dependency resolver by removing duplicates for dependency nodes.
+  [Jacek Suliga](https://github.com/jmkk)
 
 ##### Bug Fixes
 

lib/cocoapods/resolver.rb

@@ -577,7 +577,7 @@ module Pod
 
         dependency_nodes.flat_map do |item|
           valid_dependencies_for_target_from_node(target, dependencies, item)
-        end.concat dependency_nodes
+        end.concat(dependency_nodes).uniq
       end
     end