[Plugin::Index] Only register each plugin once for a given hook

lib/bundler/plugin/index.rb

@@ -58,7 +58,10 @@ def register_plugin(name, path, load_paths, commands, sources, hooks)
         raise SourceConflict.new(name, common) unless common.empty?
         sources.each {|k| @sources[k] = name }
 
-        hooks.each {|e| (@hooks[e] ||= []) << name }
+        hooks.each do |event|
+          event_hooks = (@hooks[event] ||= []) << name
+          event_hooks.uniq!
+        end
 
         @plugin_paths[name] = path
         @load_paths[name] = load_paths

spec/bundler/plugin/index_spec.rb

@@ -86,6 +86,17 @@
       expect(new_index.hook_plugins("after-bar")).to eq([plugin_name])
     end
 
+    it "only registers a gem once for an event" do
+      path = lib_path(plugin_name)
+      index.register_plugin(plugin_name,
+        path.to_s,
+        [path.join("lib").to_s],
+        commands,
+        sources,
+        hooks + hooks)
+      expect(index.hook_plugins("after-bar")).to eq([plugin_name])
+    end
+
     context "that are not registered", :focused do
       let(:file) { double("index-file") }