diff --git a/idea/idea-core/src/org/jetbrains/kotlin/idea/core/script/ScriptDefinitionsManager.kt b/idea/idea-core/src/org/jetbrains/kotlin/idea/core/script/ScriptDefinitionsManager.kt
index bc8744e81fa..788e471f491 100644
--- a/idea/idea-core/src/org/jetbrains/kotlin/idea/core/script/ScriptDefinitionsManager.kt
+++ b/idea/idea-core/src/org/jetbrains/kotlin/idea/core/script/ScriptDefinitionsManager.kt
@@ -198,7 +198,7 @@ class ScriptDefinitionsManager(private val project: Project) : LazyScriptDefinit
 
         val newExtensions = getKnownFilenameExtensions().filter {
             fileTypeManager.getFileTypeByExtension(it) != KotlinFileType.INSTANCE
-        }.toList()
+        }.toSet()
 
         if (newExtensions.any()) {
             // Register new file extensions