[DSL] Raise when specifying multiple post_install hooks

CHANGELOG.md

@@ -21,6 +21,10 @@
   [Samuel Giddins](https://github.com/segiddins)
   [#328](https://github.com/CocoaPods/Core/issues/328)
 
+* Specifying multiple `post_install` hooks in a Podfile raises an error.  
+  [Daniel Tomlinson](https://github.com/dantoml)
+  [#334](https://github.com/CocoaPods/Core/pull/334)
+
 ##### Bug Fixes
 
 * None.  

lib/cocoapods-core/podfile/dsl.rb

@@ -741,6 +741,7 @@ def pre_install(&block)
       # @return   [void]
       #
       def post_install(&block)
+        raise Informative, 'Specifying multiple `post_install` hooks is unsupported.' if @post_install_callback
         @post_install_callback = block
       end
     end

spec/podfile/dsl_spec.rb

@@ -445,6 +445,20 @@ module Pod
         end.post_install!(:an_installer)
         yielded.should == :an_installer
       end
+
+      it 'raises when post_install is called multiple times' do
+        should.raise(Informative) do
+          Podfile.new do
+            target 'App' do
+              post_install do |installer|
+              end
+
+              post_install do |installer|
+              end
+            end
+          end
+        end.message.should == 'Specifying multiple `post_install` hooks is unsupported.'
+      end
     end
 
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