[LazySpecification] Warn when there are multiple sources per version in #all_specifications

Addresses #3 of https://github.com/CocoaPods/CocoaPods/pull/2637#issuecomment-58584402

lib/cocoapods/resolver/lazy_specification.rb

 module Pod
-  class Specification::Set
-    class LazySpecification < BasicObject
-      attr_reader :name, :version, :source
+  class Specification
+    class Set
+      class LazySpecification < BasicObject
+        attr_reader :name, :version, :source
 
-      def initialize(name, version, source)
-        @name = name
-        @version = version
-        @source = source
-      end
+        def initialize(name, version, source)
+          @name = name
+          @version = version
+          @source = source
+        end
 
-      def method_missing(method, *args, &block)
-        specification.send(method, *args, &block)
-      end
+        def method_missing(method, *args, &block)
+          specification.send(method, *args, &block)
+        end
 
-      def subspec_by_name(name = nil)
-        if !name || name == self.name
-          self
-        else
-          specification.subspec_by_name(name)
+        def subspec_by_name(name = nil)
+          if !name || name == self.name
+            self
+          else
+            specification.subspec_by_name(name)
+          end
         end
-      end
 
-      def specification
-        @specification ||= source.specification(name, version)
-      end
+        def specification
+          @specification ||= source.specification(name, version)
+        end
     end
 
-    class External
-      def all_specifications
-        [specification]
+      class External
+        def all_specifications
+          [specification]
+        end
       end
-    end
 
-    def all_specifications
-      @all_specifications ||= versions_by_source.map do |source, versions|
-        versions.map { |version| LazySpecification.new(name, version, source) }
-      end.flatten
+      def all_specifications
+        @all_specifications ||= begin
+          versions_by_source.reduce({}) do |sources_by_version, (source, versions)|
+            versions.each { |v| (sources_by_version[v] ||= []) << source }
+            sources_by_version
+          end.select { |_version, sources| sources.count > 1 }.each do |version, sources|
+            UI.warn "Found multiple specifications for `#{name} (#{version})`:\n" +
+              sources.
+                map { |s| s.specification_path(name, version) }.
+                map { |v| "- #{v}" }.join("\n")
+          end
+
+          versions_by_source.map do |source, versions|
+            versions.map { |version| LazySpecification.new(name, version, source) }
+          end.flatten
+        end
+      end
     end
   end
 end