In written text, sentence repetition will stand out to your reader. This includes full and partial sentence repetition, such as the repetition of a verb or phrase. A famous example of this can be seen in Winston Churchill's speech before the Battle of Britain, in which the phrase "we shall" is repeated numerous times to strongly define the American manifesto. In many cases, sentence or phrasal repetition can make your writing seem amateur or unrefined, especially if the reader perceives the repetition as accidental. Furthermore, repeating a phrase for emphasis can sometimes have the opposite effect. Readers may pick up on the repetition and resist the point you are trying to emphasize if they feel it is being made too forcibly.
The advantages and disadvantages of sentence repetition in spoken word are similar to those of written text. However, with speech, repeated sentences do not stand out as much as when written, as listeners are likely to retain less of the speech than when reading a written text. Nevertheless, if listeners pick up on a sentence repeated for emphasis in a speech, they may instead perceive it as the product of an amateur speaking ability or a lack of speech preparation.
No matter which medium you are working in, a disadvantage of repeating sentences is the development of accidental repetition. When speaking, the continual repetition of a sentence or phrase is likely to develop into a habit. Looking back on a speech, you may find that you subconsciously repeated the sentence more times than you intended, which may make your speech seem unorganized or undeveloped. The same is true in written text. To avoid this, read over your texts in a quick manner once they are finished. This will make repeated sentences or phrases stand out, at which point you can decide if they are rhetorically effective or not.
A way to avoid the disadvantages of sentence repetition is to use different wording each time a sentence is repeated. Use synonyms and rearrange the sentence construction so that it conveys the same point while using different wording. Though such repetition will not stand out as blatantly as verbatim repetition, it will nevertheless emphasize the point you wish to make to your readers or listeners. They will likely pick up on the point more thoroughly, while not perceiving the repetition as accidental or awkward.