Jens points to a good reason for tweeting more than once across various time frames. Chances are pretty good that nobody is going to see all three of them if they're spread out across several hours.

The other side of that coin is that if all you're doing is spewing your content and not engaging in conversation, you're off my time line. The twitter dump is sometimes inadvertant -- as in a feed to twitter from some social spaces (like Ning) that updates once or twice a day with new content. Not a great idea, but I understand the limitations of the system and I'll over look it if it doesn't take up too much of my timeline.

The point of twitter is to engage, not broadcast. The degree to which that happens will be directly related to your success in the channel.

Repeating identical tweets used to violate Twitter's TOS, but they recently revised the rule to say that it's just one of the factors they consider when classifying spammers (http://help.twitter.com/entries/18311-the-twitter-rules). So sure, there are probably things that need to be communicated multiple times, but I recommend coming up with several variations on the theme.

As for prolific tweeting, as long as it's not automated, it actually works in your favor by making you look like a real person... as long as it's meaningful and not always the infamous "just ate a ham sandwich." It also helps to keep those repetitive tweets from appearing all in a row on your timeline.

All of this addresses the letter of the law, but I agree with Nathan that the intent is even more important. We can't confuse broadcasting with conversation. People are smart. If they can see that you're only interested in talking AT them, they'll just unfollow you.

I sometimes re-tweet the same link, if I've written something that I want to really get out there. If someone is following thousands of accounts, they might miss it. However, I will change the wording around the link, so I'm not tweet spamming.

I hope it's okay to answering by linking to a post explaining this practice: http://holykaw.alltop.com/the-art-of-the-repeat-tweet

I agree with Kate, re-word each tweet slightly so as not to piss off the people that follow you, but there's no harm in re-tweeting the same content, written in a new way. If your business follows a certain niche or idea, or a certain concept, your tweets will inherently follow the same ideas over and over, sometimes it's just a matter of re-tweeting them for the current story of the day, or major world event.