It's a common truism, familiar to most people by now thanks to advertising and politics, that repeating things makes them more believable -- regardless of whether they're true or not. In fact, even if they know at the time that the information is false, people will still be more likely to believe something the more they hear it. This phenomenon, sometimes called the reiteration effect, is well-studied and well-documented.
Believing things you've heard repeatedly could make sense:
[T]he more sources there are the more unlikely it seems that all of them believe it if it's false. This strategy makes some evolutionary and statistical sense. Hearing (or experiencing) something from two independent sources (or two independent events) makes it more likely that you can generalize on them than if you only experienced it once.
However, we're in an evolutionarily novel environment:
Unfortunately, in the mass media today few sources of information are independent. Most media outlets get things from AP wire services and most people get their information from the same media outlets, so even if you hear item X in 20 completely different contexts, chances are that all 20 of them stem from the same one or two original reports. If you've ever been the source of national press yourself, you will have experienced this firsthand.
Looks like I've got another rationale for skipping commercials and ignoring current events!