Timing is one of the most important parts of getting "hits" on social media, and while it can be optimized for, it's not always controllable. A post may succeed because someone made an attention-getting comment, for example, and that's just the luck of the commenter running across the link when he or she had the time and inclination to leave a note.

Social is fickle that way, and since most social algorithms strongly consider post recency and other time-sensitive factors in their ranking, duplicates should be allowed within a reasonable time frame, because you never know when the critical path will get hit.