diff --git a/src/Composer/DependencyResolver/PoolOptimizer.php b/src/Composer/DependencyResolver/PoolOptimizer.php
index 841a2e9c6..ef17938b5 100644
--- a/src/Composer/DependencyResolver/PoolOptimizer.php
+++ b/src/Composer/DependencyResolver/PoolOptimizer.php
@@ -340,6 +340,11 @@ class PoolOptimizer
      */
     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) {