Twitter can be a firehose. The sheer volume of tweets coming in every minute is staggering. So let's say you post something at 9:03am. Someone logs in to check Twitter at 9:25am. Your post could be long gone in their stream! They might have so many thousands of posts to read through that they never get to yours.
That is why it is wholly legitimate to make a fresh post of that entry a few hours later. It's not that you're spamming. It's that many people simply never saw that post.
To be fair, if you just keep making the Exact Same Post every five minutes you are going to drive people absolutely insane. It's good to mix things up. Make a promo post for your book. Then post about your writing challenge. Later on, post about your cat curling up on your lap. An hour later, post about the daffodils coming up in your yard. Then eventually tweet about the book again. It should seem reasonably new at that point.
It also helps to mix up the hashtags you use each time. That way you reach a new set of hashtag readers each time. Although the same issue with hashtag readers happens here. If someone is watching #lowcarb and they check in at 10am, they might never see your post. If you repost it fresh at 4pm and they then log in at 4:15pm, they might see that new post.
So the key here is balance.
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