diff --git a/current-api.h b/current-api.h
index d5861d0..4e32a76 100644
--- a/current-api.h
+++ b/current-api.h
@@ -199,7 +199,13 @@ void proxmox_backup_register_image_async(ProxmoxBackupHandle *handle,
  *
  * Upload a chunk of data for the <dev_id> image.
  *
- * data may be NULL in order to write the zero chunk (only allowed if size == chunk_size)
+ * The data pointer may be NULL in order to write the zero chunk
+ * (only allowed if size == chunk_size)
+ *
+ * Returns:
+ * -1: on error
+ *  0: successful, chunk already exists on server, so it was resued
+ *  size: successful, chunk uploaded
  */
 int proxmox_backup_write_data(ProxmoxBackupHandle *handle,
                               uint8_t dev_id,
@@ -218,6 +224,11 @@ int proxmox_backup_write_data(ProxmoxBackupHandle *handle,
  *
  * Note: The data pointer needs to be valid until the async
  * opteration is finished.
+ *
+ * Returns:
+ * -1: on error
+ *  0: successful, chunk already exists on server, so it was resued
+ *  size: successful, chunk uploaded
  */
 void proxmox_backup_write_data_async(ProxmoxBackupHandle *handle,
                                      uint8_t dev_id,
diff --git a/src/commands.rs b/src/commands.rs
index bee1b11..78cf0cf 100644
--- a/src/commands.rs
+++ b/src/commands.rs
@@ -274,12 +274,15 @@ pub(crate) async fn write_data(
         (info.wid, info.upload_queue.clone(), info.zero_chunk_digest)
     };
 
+    let mut reused = false;
+
     let upload_future: Box<dyn Future<Output = Result<ChunkUploadInfo, Error>> + Send + Unpin> = {
         if data.0.is_null() {
             if size != chunk_size {
                 bail!("write_data: got invalid null chunk");
             }
             let upload_info = ChunkUploadInfo { digest: zero_chunk_digest, offset, size, chunk_is_known: true };
+            reused = true;
             Box::new(futures::future::ok(upload_info))
         } else {
             let data: &[u8] = unsafe { std::slice::from_raw_parts(data.0, size as usize) };
@@ -299,6 +302,7 @@ pub(crate) async fn write_data(
 
             if chunk_is_known {
                 let upload_info = ChunkUploadInfo { digest: *digest, offset, size, chunk_is_known: true };
+                reused = true;
                 Box::new(futures::future::ok(upload_info))
            } else {
                 let (chunk, digest) = chunk_builder.build()?;
@@ -363,7 +367,7 @@ pub(crate) async fn write_data(
 
     //println!("upload chunk sucessful");
 
-    Ok(size as c_int)
+    Ok(if reused { 0 } else { size as c_int })
 }
 
 pub(crate) async fn finish_backup(
diff --git a/src/lib.rs b/src/lib.rs
index fb52ead..7d3d471 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -387,7 +387,13 @@ pub extern "C" fn proxmox_backup_add_config_async(
 ///
 /// Upload a chunk of data for the <dev_id> image.
 ///
-/// data may be NULL in order to write the zero chunk (only allowed if size == chunk_size)
+/// The data pointer may be NULL in order to write the zero chunk
+/// (only allowed if size == chunk_size)
+///
+/// Returns:
+/// -1: on error
+///  0: successful, chunk already exists on server, so it was resued
+///  size: successful, chunk uploaded
 #[no_mangle]
 #[allow(clippy::not_unsafe_ptr_arg_deref)]
 pub extern "C" fn proxmox_backup_write_data(
@@ -426,6 +432,11 @@ pub extern "C" fn proxmox_backup_write_data(
 ///
 /// Note: The data pointer needs to be valid until the async
 /// opteration is finished.
+///
+/// Returns:
+/// -1: on error
+///  0: successful, chunk already exists on server, so it was resued
+///  size: successful, chunk uploaded
 #[no_mangle]
 #[allow(clippy::not_unsafe_ptr_arg_deref)]
 pub extern "C" fn proxmox_backup_write_data_async(