Sometimes in life we get irritated at people who keep saying the same thing again and again.
However, I think if you have an important scientific message you need to realise that you may need to keep repeating it. If you have something original and/or profound to say it is not going to be easy for people to grasp and/or accept. With some talks I have no idea what the speaker is trying to say. Talks that end with 10 conclusions don't help! Even for good speakers I find I benefit from hearing the talk several times in different forums over a period of time, interspersed with looking at the relevant papers, and sometimes blogging about them.
I am not alone. I have noticed that even after someone has heard one of my talks several times in different forums and I have talked informally with them about it, there are basic points they still don't appreciate or get on the third hearing.
Action point: don't be shy about giving a similar talk to one you have given before. Try to have just one message, not several. Talking about several topics almost never works.
Similar issues are relevant with teaching. Just telling students something once usually has no impact. They need to hear it several times. Furthermore, encountering the same idea from different sources and through different media may be necessary for the idea, concept, or technique to be understood, appreciated, and mastered. Different media include lectures, tutorials, text books, computer simulations, quizzes, and peer instruction. Furthermore, having different people explain something in different ways can be quite helpful. This is one reason when I teach a class I often review material that someone else may have already "taught" them. This is not because I think I am a better teacher than my colleague. I think students benefit from hearing the same material from someone else and review can be beneficial. I do this because students tell me it is helpful.