Early return when a package was already marked to be kept

Author: Toflar

src/Composer/DependencyResolver/PoolOptimizer.php

@@ -340,6 +340,11 @@ private function markPackageForRemoval($id)
      */
     private function keepPackage(BasePackage $package, $identicalDefinitionsPerPackage, $packageIdenticalDefinitionLookup)
     {
+        // Already marked to keep
+        if (!isset($this->packagesToRemove[$package->id])) {
+            return;
+        }
+
         unset($this->packagesToRemove[$package->id]);
 
         if ($package instanceof AliasPackage) {