When looking to convince someone that what you are telling them is the truth, bear in mind that we tend to trust things that feel familiar to us. Therefore, telling someone something that you have previously shared with them becomes more convincing through its repetition.
Research shows that people are likelier to believe eyewitness statements when they are repeated, even if it was the same person restating a lie. We often work hard to ensure that we don't repeat ourselves, yet it seems that there will be times where repetition may serve our interests better.
Familiarity can lead to acceptance... say it twice!