"I'm only going to tell you this once…"
There's a lot to be said for conditioning your audience to listen carefully. If they know that valuable information is only going to come at them once, they'll be more alert for it.
Alas, as the nois-o-sphere gets noisier still, this approach is hard to justify.
Repetition increases the chance that you get heard.
Repetition also increases (for a while) the authority and believability of what you have to say. Listeners go from awareness of the message to understanding to trust. Yes, the step after that is annoyance, which is the risk the marketer always faces.
Delivering your message in different ways, over time, not only increases retention and impact, but it gives you the chance to describe what you're doing from several angles.
In many ways, the mantra of permission conflicts with the mechanics of frequency. If people are loaning you their attention and you're delivering anticipated, personal and relevant messages, your need for frequency goes way down.
If you're using frequency as a tactic to make up for the fact that you're being ignored, you can certainly do better.
For the rest of us though, saying it twice may in fact be twice as good as saying it once.