diff --git a/pro/src/conflictLogic.ts b/pro/src/conflictLogic.ts
index c19a7a7..36eca36 100644
--- a/pro/src/conflictLogic.ts
+++ b/pro/src/conflictLogic.ts
@@ -200,22 +200,141 @@ export function getFileRename(key: string) {
   return res;
 }
 
+function arraysAreEqual(arr1: ArrayBuffer, arr2: ArrayBuffer) {
+  if (arr1.byteLength !== arr2.byteLength) {
+    return false;
+  }
+  const u1 = new Uint8Array(arr1);
+  const u2 = new Uint8Array(arr2);
+
+  for (let i = 0; i < u1.byteLength; ++i) {
+    if (u1[i] !== u2[i]) {
+      return false;
+    }
+  }
+
+  return true;
+}
+
+/**
+ * 1. download remote
+ * 2. compare
+ * 3. if the same, update local but not upload
+ * 4. if not the same, rename local and save remote
+ */
+async function tryDuplicateFileForSameSizes(
+  key: string,
+  key2: string,
+  fsLocal: FakeFs,
+  fsRemote: FakeFs,
+  uploadCallback: (entity: Entity | undefined) => Promise<any>,
+  downloadCallback: (entity: Entity | undefined) => Promise<any>
+) {
+  console.debug(`tryDuplicateFileForSameSizes: ${key}`);
+
+  // 1. download
+  const remoteContent = await fsRemote.readFile(key);
+
+  // 2. compare
+  const localContent = await fsLocal.readFile(key);
+  const eq = arraysAreEqual(localContent, remoteContent);
+
+  if (eq) {
+    // 3. if the same, update local but not upload
+    // read meta of remote, as if we have downloaded the file
+    console.debug(`tryDuplicateFileForSameSizes: ${key} content equal`);
+    const entityRemote = await fsRemote.stat(key);
+
+    // write
+    const downloadResultEntity = await fsLocal.writeFile(
+      key,
+      remoteContent,
+      entityRemote.mtimeCli ?? Date.now(),
+      entityRemote.mtimeCli ?? Date.now()
+    );
+    await downloadCallback(downloadResultEntity);
+
+    // no uploadCallback here
+  } else {
+    // 4. if not the same, rename local and save remote
+    console.debug(`tryDuplicateFileForSameSizes: ${key} content not equal`);
+
+    await fsLocal.rename(key, key2);
+
+    const entityRemote = await fsRemote.stat(key);
+    const downloadResultEntity = await fsLocal.writeFile(
+      key,
+      remoteContent,
+      entityRemote.mtimeCli ?? Date.now(),
+      entityRemote.mtimeCli ?? Date.now()
+    );
+    await downloadCallback(downloadResultEntity);
+
+    const entityLocal = await fsLocal.stat(key2); // key2 here!
+    const uploadResultEntity = await fsRemote.writeFile(
+      key2, // key2 here!
+      localContent,
+      entityLocal.mtimeCli ?? Date.now(),
+      entityLocal.mtimeCli ?? Date.now()
+    );
+    await uploadCallback(uploadResultEntity);
+  }
+}
+
 /**
  * local: x.md -> x.dup.md -> upload to remote
  * remote: x.md -> download to local -> using original name x.md
  */
-export async function duplicateFile(
+async function tryDuplicateFileForDiffSizes(
   key: string,
-  left: FakeFs,
-  right: FakeFs,
-  uploadCallback: (entity: Entity) => Promise<any>,
-  downloadCallback: (entity: Entity) => Promise<any>
+  key2: string,
+  fsLocal: FakeFs,
+  fsRemote: FakeFs,
+  uploadCallback: (entity: Entity | undefined) => Promise<any>,
+  downloadCallback: (entity: Entity | undefined) => Promise<any>
+) {
+  console.debug(`tryDuplicateFileForDiffSizes: ${key}`);
+
+  await fsLocal.rename(key, key2);
+
+  /**
+   * x.dup.md -> upload to remote
+   */
+  async function f1() {
+    const k = await copyFile(key2, fsLocal, fsRemote);
+    await uploadCallback(k.entity);
+    return k.entity;
+  }
+
+  /**
+   * x.md -> download to local
+   */
+  async function f2() {
+    const k = await copyFile(key, fsRemote, fsLocal);
+    await downloadCallback(k.entity);
+    return k.entity;
+  }
+
+  const [resUpload, resDownload] = await Promise.all([f1(), f2()]);
+
+  return {
+    upload: resUpload,
+    download: resDownload,
+  };
+}
+
+export async function tryDuplicateFile(
+  key: string,
+  fsLocal: FakeFs,
+  fsRemote: FakeFs,
+  uploadCallback: (entity: Entity | undefined) => Promise<any>,
+  downloadCallback: (entity: Entity | undefined) => Promise<any>
 ) {
   let key2 = getFileRename(key);
   let usable = false;
   do {
     try {
-      const s = await left.stat(key2);
+      const s = await fsLocal.stat(key2);
       if (s === null || s === undefined) {
         throw Error(`not exist $${key2}`);
       }
@@ -228,30 +347,31 @@ export async function duplicateFile(
       usable = true;
     }
   } while (!usable);
-  await left.rename(key, key2);
 
-  /**
-   * x.dup.md -> upload to remote
-   */
-  async function f1() {
-    const k = await copyFile(key2, left, right);
-    await uploadCallback(k.entity);
-    return k.entity;
+  const localSize = await fsLocal.stat(key);
+  const remoteSize = await fsRemote.stat(key);
+
+  if (
+    localSize !== undefined &&
+    remoteSize !== undefined &&
+    localSize.sizeRaw === remoteSize.sizeRaw
+  ) {
+    return await tryDuplicateFileForSameSizes(
+      key,
+      key2,
+      fsLocal,
+      fsRemote,
+      uploadCallback,
+      downloadCallback
+    );
+  } else {
+    return await tryDuplicateFileForDiffSizes(
+      key,
+      key2,
+      fsLocal,
+      fsRemote,
+      uploadCallback,
+      downloadCallback
+    );
   }
-
-  /**
-   * x.md -> download to local
-   */
-  async function f2() {
-    const k = await copyFile(key, right, left);
-    await downloadCallback(k.entity);
-    return k.entity;
-  }
-
-  const [resUpload, resDownload] = await Promise.all([f1(), f2()]);
-
-  return {
-    upload: resUpload,
-    download: resDownload,
-  };
 }
diff --git a/pro/src/sync.ts b/pro/src/sync.ts
index d08d90a..2c01886 100644
--- a/pro/src/sync.ts
+++ b/pro/src/sync.ts
@@ -36,7 +36,7 @@ import {
 } from "../../src/misc";
 import type { Profiler } from "../../src/profiler";
 import { checkProRunnableAndFixInplace } from "./account";
-import { duplicateFile, isMergable, mergeFile } from "./conflictLogic";
+import { isMergable, mergeFile, tryDuplicateFile } from "./conflictLogic";
 import {
   clearFileContentHistoryByVaultAndProfile,
   getFileContentHistoryByVaultAndProfile,
@@ -1351,27 +1351,31 @@ const dispatchOperationToActualV3 = async (
         r.key
       );
       const mtimeCli = (await fsLocal.stat(r.key)).mtimeCli!;
-      const { upload, download } = await duplicateFile(
+      await tryDuplicateFile(
         r.key,
         fsLocal,
         fsEncrypt,
         async (upload) => {
-          // TODO: abstract away the dirty hack
-          fullfillMTimeOfRemoteEntityInplace(upload, mtimeCli);
-          await upsertPrevSyncRecordByVaultAndProfile(
-            db,
-            vaultRandomID,
-            profileID,
-            upload
-          );
+          if (upload !== undefined) {
+            // TODO: abstract away the dirty hack
+            fullfillMTimeOfRemoteEntityInplace(upload, mtimeCli);
+            await upsertPrevSyncRecordByVaultAndProfile(
+              db,
+              vaultRandomID,
+              profileID,
+              upload
+            );
+          }
         },
         async (download) => {
-          await upsertPrevSyncRecordByVaultAndProfile(
-            db,
-            vaultRandomID,
-            profileID,
-            download
-          );
+          if (download !== undefined) {
+            await upsertPrevSyncRecordByVaultAndProfile(
+              db,
+              vaultRandomID,
+              profileID,
+              download
+            );
+          }
         }
       );
     }