Register plugin only one time when it's present in global and project mode

src/Composer/Plugin/PluginManager.php

@@ -37,6 +37,7 @@ class PluginManager
     protected $versionParser;
 
     protected $plugins = array();
+    protected $registeredPlugins = array();
 
     private static $classCounter = 0;
 
@@ -191,6 +192,10 @@ class PluginManager
     {
         $oldInstallerPlugin = ($package->getType() === 'composer-installer');
 
+        if (in_array($package->getName(), $this->registeredPlugins)) {
+            return;
+        }
+
         $extra = $package->getExtra();
         if (empty($extra['class'])) {
             throw new \UnexpectedValueException('Error while installing '.$package->getPrettyName().', composer-plugin packages should have a class defined in their extra key to be usable.');
@@ -233,6 +238,7 @@ class PluginManager
             } else {
                 $plugin = new $class();
                 $this->addPlugin($plugin);
+                $this->registeredPlugins[] = $package->getName();
             }
         }
     }

tests/Composer/Test/Plugin/PluginInstallerTest.php

@@ -164,4 +164,21 @@ class PluginInstallerTest extends \PHPUnit_Framework_TestCase
         $plugins = $this->pm->getPlugins();
         $this->assertEquals('installer-v3', $plugins[1]->version);
     }
+
+    public function testRegisterPluginOnlyOneTime()
+    {
+        $this->repository
+            ->expects($this->exactly(2))
+            ->method('getPackages')
+            ->will($this->returnValue(array()));
+        $installer = new PluginInstaller($this->io, $this->composer);
+        $this->pm->loadInstalledPlugins();
+
+        $installer->install($this->repository, $this->packages[0]);
+        $installer->install($this->repository, clone $this->packages[0]);
+
+        $plugins = $this->pm->getPlugins();
+        $this->assertCount(1, $plugins);
+        $this->assertEquals('installer-v1', $plugins[0]->version);
+    }
 }