fix: Delete resent messages on receiver side (#5155)

If a Delta Chat message has the Message-ID already existing in the db, but a greater "Date", it's a
resent message that can be deleted. Messages having the same "Date" mustn't be deleted because they
can be already seen messages moved back to INBOX. Also don't delete messages having lesser "Date" to
avoid deleting both messages in a multi-device setting.

src/imap.rs

@@ -73,6 +73,7 @@ pub enum ImapActionResult {
 ///   not necessarily sent by Delta Chat.
 const PREFETCH_FLAGS: &str = "(UID INTERNALDATE RFC822.SIZE BODY.PEEK[HEADER.FIELDS (\
                               MESSAGE-ID \
+                              DATE \
                               X-MICROSOFT-ORIGINAL-MESSAGE-ID \
                               FROM \
                               IN-REPLY-TO REFERENCES \
@@ -769,6 +770,7 @@ impl Imap {
         let mut uids_fetch = Vec::<(_, bool /* partially? */)>::with_capacity(msgs.len() + 1);
         let mut uid_message_ids = BTreeMap::new();
         let mut largest_uid_skipped = None;
+        let delete_target = context.get_delete_msgs_target().await?;
 
         // Store the info about IMAP messages in the database.
         for (uid, ref fetch_response) in msgs {
@@ -794,8 +796,24 @@ impl Imap {
             // Such move to the same folder results in the messages
             // getting a new UID, so the messages will be detected as new
             // in the `INBOX.DeltaChat` folder again.
+            let _target;
             let target = if let Some(message_id) = &message_id {
-                if context
+                let is_dup = if let Some((_, ts_sent_old)) =
+                    message::rfc724_mid_exists(context, message_id).await?
+                {
+                    let is_chat_msg = headers.get_header_value(HeaderDef::ChatVersion).is_some();
+                    let ts_sent = headers
+                        .get_header_value(HeaderDef::Date)
+                        .and_then(|v| mailparse::dateparse(&v).ok())
+                        .unwrap_or_default();
+                    is_dup_msg(is_chat_msg, ts_sent, ts_sent_old)
+                } else {
+                    false
+                };
+                if is_dup {
+                    info!(context, "Deleting duplicate message {message_id}.");
+                    &delete_target
+                } else if context
                     .sql
                     .exists(
                         "SELECT COUNT (*) FROM imap WHERE rfc724_mid=?",
@@ -807,9 +825,10 @@ impl Imap {
                         context,
                         "Not moving the message {} that we have seen before.", &message_id
                     );
-                    folder.to_string()
+                    folder
                 } else {
-                    target_folder(context, folder, folder_meaning, &headers).await?
+                    _target = target_folder(context, folder, folder_meaning, &headers).await?;
+                    &_target
                 }
             } else {
                 // Do not move the messages without Message-ID.
@@ -819,7 +838,7 @@ impl Imap {
                     context,
                     "Not moving the message that does not have a Message-ID."
                 );
-                folder.to_string()
+                folder
             };
 
             // Generate a fake Message-ID to identify the message in the database
@@ -834,7 +853,7 @@ impl Imap {
                        ON CONFLICT(folder, uid, uidvalidity)
                        DO UPDATE SET rfc724_mid=excluded.rfc724_mid,
                                      target=excluded.target",
-                    (&message_id, &folder, uid, uid_validity, &target),
+                    (&message_id, &folder, uid, uid_validity, target),
                 )
                 .await?;
 
@@ -887,6 +906,7 @@ impl Imap {
                     .fetch_many_msgs(
                         context,
                         folder,
+                        uid_validity,
                         uids_fetch_in_batch.split_off(0),
                         &uid_message_ids,
                         fetch_partially,
@@ -1431,10 +1451,12 @@ impl Imap {
     /// Returns the last UID fetched successfully and the info about each downloaded message.
     /// If the message is incorrect or there is a failure to write a message to the database,
     /// it is skipped and the error is logged.
+    #[allow(clippy::too_many_arguments)]
     pub(crate) async fn fetch_many_msgs(
         &mut self,
         context: &Context,
         folder: &str,
+        uidvalidity: u32,
         request_uids: Vec<u32>,
         uid_message_ids: &BTreeMap<u32, String>,
         fetch_partially: bool,
@@ -1568,6 +1590,9 @@ impl Imap {
                 );
                 match receive_imf_inner(
                     context,
+                    folder,
+                    uidvalidity,
+                    request_uid,
                     rfc724_mid,
                     body,
                     is_seen,
@@ -2247,6 +2272,15 @@ pub(crate) async fn prefetch_should_download(
     Ok(should_download)
 }
 
+/// Returns whether a message is a duplicate (resent message).
+pub(crate) fn is_dup_msg(is_chat_msg: bool, ts_sent: i64, ts_sent_old: i64) -> bool {
+    // If the existing message has timestamp_sent == 0, that means we don't know its actual sent
+    // timestamp, so don't delete the new message. E.g. outgoing messages have zero timestamp_sent
+    // because they are stored to the db before sending. Also consider as duplicates only messages
+    // with greater timestamp to avoid deleting both messages in a multi-device setting.
+    is_chat_msg && ts_sent_old != 0 && ts_sent > ts_sent_old
+}
+
 /// Marks messages in `msgs` table as seen, searching for them by UID.
 ///
 /// Returns updated chat ID if any message was marked as seen.