Repetition with purpose can be one of the most powerful things you can do. Repetition without purpose can be deadly.
To avoid boring your audience, you should vary your words, not repeating any particular word more than necessary. (This also applies to the pace at which you speak, your gestures, your pitch and volume, and so on. Variety enhances your presentations.)
On the other hand, deliberate repetition can be powerful. Consider —
“I hate flies. House flies. Horse flies. Even pop flies.”
Or Lincoln's "... government of the people, by the people, for the people..."
Kennedy's "... ask not what your country can do for you, ask what you can do for your country" not only repeats his words, but reverses them — another powerful technique.
So repetition is an effective way to bring emphasis to your presentation. But repetition without purpose gets boring quickly, so a powerful vocabulary can help you avoid excessive repetition of some words. And if there’s a word that you must use repeatedly, try saying it with different inflections or in different contexts — anything to avoid putting your audience into a trance.