Two negatives used together in a sentence constitute a double negative. The use of a double negative to express a positive is acceptable, although it yields a weaker affirmative than the simpler positive and may be confusing:
Our results are not inconsistent with the prior hypothesis.
More direct incentives have produced substantial changes in behavior in the past, although not without adverse consequences.
Rheumatologic symptoms were not uncommon in both groups.
However, it is not grammatically acceptable to use a double negative to emphasize the negative. In the following example, the double negative conveys the opposite of what is intended.
The authors cannot barely contain their enthusiasm.
A double negative is best avoided in scientific writing because it often causes the reader to go back and reread the sentence to make sure of the meaning.