C# Entity Framework set referenced entity to new parent entity

I've got a list of messages which are referenced by a command. This command can succeed or fail. Every command should be a new entity, but the message should be unique, it should always reference to the last command or to the successful command. So I've tried to delete the old message, I deleted the reference between command and message but always get this error:

The instance of entity type 'DbMessage' cannot be tracked because another instance with the same key value for {'Id'} is already being tracked. When attaching existing entities, ensure that only one entity instance with a given key value is attached. Consider using 'DbContextOptionsBuilder.EnableSensitiveDataLogging' to see the conflicting key values.

To ensure it's the same message, I use this method:

public IDbMessage FirstOrdDefaultMessage(Func<IDbMessage, bool> predicate)
{
    var attached = _context.Messages.Local.FirstOrDefault(predicate);

    if (attached == null)
        attached = _context.Messages.FirstOrDefault(predicate);

    return attached;
}

But I still get the error. It even returns the message on local but only once but there should be two.