diff --git a/packages/react-reconciler/src/ReactFiberWorkLoop.js b/packages/react-reconciler/src/ReactFiberWorkLoop.js
index 5fafd8de5c903..d045fa08496e1 100644
--- a/packages/react-reconciler/src/ReactFiberWorkLoop.js
+++ b/packages/react-reconciler/src/ReactFiberWorkLoop.js
@@ -3792,12 +3792,9 @@ function commitDoubleInvokeEffectsInDEV(
   hasPassiveEffects: boolean,
 ) {
   if (__DEV__) {
-    if (useModernStrictMode) {
+    if (useModernStrictMode && root.tag !== LegacyRoot) {
       let doubleInvokeEffects = true;
 
-      if (root.tag === LegacyRoot && !(root.current.mode & StrictLegacyMode)) {
-        doubleInvokeEffects = false;
-      }
       if (
         root.tag === ConcurrentRoot &&
         !(root.current.mode & (StrictLegacyMode | StrictEffectsMode))
diff --git a/packages/shared/ReactFeatureFlags.js b/packages/shared/ReactFeatureFlags.js
index 67862a4f14288..3d8a7cf563bc8 100644
--- a/packages/shared/ReactFeatureFlags.js
+++ b/packages/shared/ReactFeatureFlags.js
@@ -147,7 +147,7 @@ export const disableJavaScriptURLs = false;
 // Modern <StrictMode /> behaviour aligns more with what components
 // components will encounter in production, especially when used With <Offscreen />.
 // TODO: clean up legacy <StrictMode /> once tests pass WWW.
-export const useModernStrictMode = false;
+export const useModernStrictMode = __NEXT_MAJOR__;
 
 // Not ready to break experimental yet.
 // Remove IE and MsApp specific workarounds for innerHTML