Some writing teachers like to expound on the value of repetition.  Repeating ideas and keywords, after all, is one of way of ensuring your reader never forgets about them.  On the flipside, however, their effect is not always positive.

The same word or phrase repeated too closely is distracting.  Unless it's being used for effect (such as emphasis or irony), it will usually leave the reader pausing, trying to recognize any significance to the repetition.  Plus, it reads very awkwardly.

Have you seen web copy that tries to achieve unusually high keyword density?  Yep, it reads like it was  written by a second-language speaker with a whole three weeks of English learning in his arsenal.  It sounds awful and ridiculous.  Please stay away from that.

One problem is that doing a simple software-based grammar check will not identify this.  For most types of composition tools, you'll have to configure the program to watch out for style issues, which excess repetition is.

Similarly, repeating the same idea can feel tedious when you're not adding anything new.  The trick is to reinforce it by a secondary thought, rather than merely parroting for the sake of repetition.  A five paragraph essay, for instance, with a single idea repeated over and over without putting any new arguments to the table will probably work better shaved down into a couple hundred words.   You don't have to repeat anything since you only have one thought.  Just state that and end it - you'll make more of an impact that way.