When you are using your email campaign to promote a product it is worth sending out a similar promotional email a few times. The reason is because not everyone will open your first email.
However, there will be times when someone doesn't need your product now, but they might need it in a few months time. So not only should you send a couple of repeat emails in a row about a product you are promoting, you should also send one out again a few months later and at regular intervals.
Another reason why you should send repeat promotional news emails is because some people won't buy unless they have seen your sales page a few times. If some subscribers buy your product the first time you send out your emails you will notice that other subscribers purchase after you send the second email.
If people are still buying your product it might be worth sending out another email until no one buys it. Keep a spreadsheet and see if there is a pattern in terms of the optimal number of emails to obtain the highest number of sales.
The purpose of your email campaign is ultimately to build trust, build your credibility and to communicate your business values. Therefore you need to test your campaign when you send repeat emails to make sure that people are still opening your emails and that the click through rate is still high.
Any dramatic changes in these rates could indicate that your subscribers like or dislike your repeat emails. However, you need to balance that with the number of sales you are making by sending them.