diff --git a/docs/algorithm.md b/docs/algorithm.md
index efd6dc4..315a8d7 100644
--- a/docs/algorithm.md
+++ b/docs/algorithm.md
@@ -14,14 +14,16 @@ Assuming all sources are reliable.
 
 We list all combinations mutually exclusive and collectively exhaustive.
 
-| ID  | Remote Files | Local files | Local delete rename history | Extra                                           | Decision                                                                                          |
-| --- | ------------ | ----------- | --------------------------- | ----------------------------------------------- | ------------------------------------------------------------------------------------------------- |
-| 1   | exist        | exist       | ignore                      | mtime_remote > mtime_local                      | download remote file, create local folder if not exists, clear local history if exists            |
-| 2   | exist        | exist       | ignore                      | mtime_remote <= mtime_local                     | upload local file, create remote folder if not exists, clear local history if exists              |
-| 3   | exist        | exist       | ignore                      | If local is a folder. mtime_local === undefined | clear local history if exists. TODO: what if a folder and a previous file share the same name?    |
-| 4   | exist        | not exist   | exist                       | mtime_remote >= delete_time_local               | download remote file, create folder if not exists                                                 |
-| 5   | exist        | not exist   | exist                       | mtime_remote < delete_time_local                | delete remote file, clear local history                                                           |
-| 6   | exist        | not exist   | not exist                   |                                                 | download remote file, create folder if not exists                                                 |
-| 7   | not exist    | exist       | ignore                      | If local is a single file.                      | upload local file, create remote folder if not exists, clear local history if exists              |
-| 8   | not exist    | exist       | ignore                      | If local is a folder.                           | upload local files recursively, create remote folder if not exists, clear local history if exists |
-| 9   | not exist    | not exist   | ignore                      |                                                 | clear local history if exists                                                                     |
+| ID  | Remote Files | Local files | Local delete rename history | Extra                                                      | Decision                                                                                          |
+| --- | ------------ | ----------- | --------------------------- | ---------------------------------------------------------- | ------------------------------------------------------------------------------------------------- |
+| 1   | exist        | exist       | ignore                      | mtime_remote > mtime_local                                 | download remote file, create local folder if not exists, clear local history if exists            |
+| 2   | exist        | exist       | ignore                      | mtime_remote < mtime_local                                 | upload local file, create remote folder if not exists, clear local history if exists              |
+| 3   | exist        | exist       | ignore                      | mtime_remote === mtime_local && size_remote === size_local | clear local history if exists (the file was synced and no changes after last sync)                |
+| 4   | exist        | exist       | ignore                      | mtime_remote === mtime_local && size_remote !== size_local | upload local file, clear local history if exists (we always prefer local to remote)               |
+| 5   | exist        | exist       | ignore                      | If local is a folder. mtime_local === undefined            | clear local history if exists. TODO: what if a folder and a previous file share the same name?    |
+| 6   | exist        | not exist   | exist                       | mtime_remote >= delete_time_local                          | download remote file, create folder if not exists                                                 |
+| 7   | exist        | not exist   | exist                       | mtime_remote < delete_time_local                           | delete remote file, clear local history                                                           |
+| 8   | exist        | not exist   | not exist                   |                                                            | download remote file, create folder if not exists                                                 |
+| 9   | not exist    | exist       | ignore                      | If local is a single file.                                 | upload local file, create remote folder if not exists, clear local history if exists              |
+| 10  | not exist    | exist       | ignore                      | If local is a folder.                                      | upload local files recursively, create remote folder if not exists, clear local history if exists |
+| 11  | not exist    | not exist   | ignore                      |                                                            | clear local history if exists                                                                     |
diff --git a/src/sync.ts b/src/sync.ts
index 0eaed27..4d168ba 100644
--- a/src/sync.ts
+++ b/src/sync.ts
@@ -28,6 +28,8 @@ interface FileOrFolderMixedState {
   mtime_local?: number;
   mtime_remote?: number;
   delete_time_local?: number;
+  size_local?: number;
+  size_remote?: number;
   decision?: DecisionType;
   syncDone?: "done";
 }
@@ -46,11 +48,13 @@ export const ensembleMixedStates = (
       key: key,
       exist_remote: true,
       mtime_remote: entry.LastModified.valueOf(),
+      size_remote: entry.Size,
     };
     if (results.hasOwnProperty(key)) {
       results[key].key = r.key;
       results[key].exist_remote = r.exist_remote;
       results[key].mtime_remote = r.mtime_remote;
+      results[key].size_remote = r.size_remote;
     } else {
       results[key] = r;
     }
@@ -68,6 +72,7 @@ export const ensembleMixedStates = (
         key: entry.path,
         exist_local: true,
         mtime_local: entry.stat.mtime,
+        size_local: entry.stat.size,
       };
     } else if (entry instanceof TFolder) {
       key = `${entry.path}/`;
@@ -75,6 +80,7 @@ export const ensembleMixedStates = (
         key: key,
         exist_local: true,
         mtime_local: undefined,
+        size_local: 0,
       };
     } else {
       throw Error(`unexpected ${entry}`);
@@ -84,6 +90,7 @@ export const ensembleMixedStates = (
       results[key].key = r.key;
       results[key].exist_local = r.exist_local;
       results[key].mtime_local = r.mtime_local;
+      results[key].size_local = r.size_local;
     } else {
       results[key] = r;
     }
@@ -156,7 +163,25 @@ export const getOperation = (
     r.exist_local &&
     r.mtime_remote !== undefined &&
     r.mtime_local !== undefined &&
-    r.mtime_remote <= r.mtime_local
+    r.mtime_remote < r.mtime_local
+  ) {
+    r.decision = "upload_clearhist";
+  } else if (
+    r.exist_remote &&
+    r.exist_local &&
+    r.mtime_remote !== undefined &&
+    r.mtime_local !== undefined &&
+    r.mtime_remote === r.mtime_local &&
+    r.size_local === r.size_remote
+  ) {
+    r.decision = "skip";
+  } else if (
+    r.exist_remote &&
+    r.exist_local &&
+    r.mtime_remote !== undefined &&
+    r.mtime_local !== undefined &&
+    r.mtime_remote === r.mtime_local &&
+    r.size_local === r.size_remote
   ) {
     r.decision = "upload_clearhist";
   } else if (