onNewMailReceived Event returns message list containing messages from previous

Question

Since the filtering is not the best I wanted to implement my own filter.

browser.messages.onNewMailReceived.addListener(function(folder, messageList) {

if(folder.accountId == "account1") {
    if(folder.path == "/INBOX") {

        console.log("Folder", typeof folder, folder);
        console.log("MessageList", typeof messageList, messageList);

        let messages = messageList.messages;

        for(let i = 0; i < messages.length; i++) {
            
            let message = messages[i];

            console.log("Message", typeof message, message);
            message.flagged = true;
        }

        messageList.messages = [];
    } else {    
        // Ignored not in inbox
    }
} else {
    // Ignored
}
});

I Expected that everytime I recieve a new message, the 'messageList' only contains new messages. But it will also contain messages from previous events. Is it a normal behaviour, do I need to mark the message as "was processd by this handler" or something like that, is it an error in the API?