Because Echo posts are scheduled to post to your Queue, these posts are sent to your Queue before they are published. If your Queue is full at the time of the original post, the Echo post will not be scheduled to your Queue, but instead, discarded. Some Socials also do reject duplicate Content within a certain period of time. If this is the case, you will see an error in your Social error log that indicates that the post was rejected.
Please note that Twitter Echo posts do not go out as original Tweets, but will instead go out as Retweets.