Sometimes, people seem to think that the more they repeat an idea, the more likely it is that someone else will believe it. In other words, they are trying to convince people of something not based upon reasons or evidence, but instead upon sheer repetition. But why do some think that such a tactic will work?
Probably because it does its a common feature of both advertising and politics to repeat the same idea over and over until people believe it. Sometimes it happens quickly and sometimes it takes longer, but in the end propagandists can get quite a lot of mileage out of repetition.
Sometimes this tactic is referred to as argumentum ad nauseam argument to nausea, based upon the idea that the position is repeated until people become sick of it. Other times, when this occurs not in the context of a single individuals arguments but instead in the context of a wider culture, it is referred to as Communal Reinforcement.
In the latter situation, the mass media often plays an important role in the spread of invalid ideas, for example by failing to provide any skeptical arguments or even acknowledging that skepticism exists. When was the last time that a report alien abductions or spiritualism spent anywhere close to the same amount of time on skeptical perspectives? When was the last time an article or program on religion offered any time to nonbelievers?
The reason why this is a flaw in reasoning is that the validity or truth of an idea has absolutely nothing whatsoever to do with how often the idea is repeated. You can repeat something dozens of times and it will still be false, while a truth remains a truth even if it is stated only once. Because of this, it is better to avoid repetition as much as possible.
This is not to say, of course, that repetition is always wrong. Sometimes you will find that people dont entirely understand what you are trying to say. Sometimes you might be legitimately concerned that the primary way you have phrased an idea wont be understood. In either case, it would not be improper for you to repeat your basic point in different words.
It is also valid to repeat your main point in order to make sure that people remember it. A common piece of advice for formal debating and public speaking is to Tell em what youre gonna tell em, then tell em, and then tell em what you told em. So long as this is accompanied by good reasons to believe that your central point is true, there shouldnt be anything wrong with this.
Of course, even that can become overbearing if it goes too far: it may not be fallacious to repeat the truth multiple times and in different forms, but it can get annoying over time.
If you encounter someone engaging in argumentum ad nauseam, the best course of action is probably to just point out how often they have repeated their assertion and how little you have seen in the way of support for that assertion. Acknowledge that it is possible that they are right, but explain to them that that cannot be established simply through repetition; instead, you need sound evidence and rational arguments in order to reach that conclusion.
If you encounter a belief or argument which appears to have been created through Communal Reinforcement, your task might be a bit more difficult. When you ask for support for the claim, a person can readily cite all kinds of sources in the media and literature where this claim has been repeated. What you will need to do is explain how and why the repetition of a claim in no way validates it; rather, you need independent support, evidence, and/or arguments which show that it is more likely true than false.