Is this double negative acceptable?

"Such peer pressure is not uncommon among Harry Potter readers". Is this double negative acceptable?
Not only is it absolutely acceptable, it is also widely used. Negative structures, including double negatives, have the effect of adding emphasis to what we want to express. Consider these two sentences：1/ This report is not to be released to the public under any circumstances; 2/ Under no circumstances is this report to be released to the public. Obviously the 2nd sentence is more forceful or persuasive than the 1st one. If you pay attention to what you read and hear, you should easily come across negative structures in English.
It happens that I also make use of double negatives in my writing. A few years ago, I attended a course Academic Writing Skills at HKU SPACE, and the tutor was a native English speaker. The course required students to write essays using various techniques. In one of my essays "Should the Death Penalty be Abolished in mainland China?" I wrote：'..... Another argument for the abolition of capital punishment on the mainland is that the judicial system has long been criticized as unsound, and miscarriages of justice are not uncommon' (支持內地廢除死刑的另一理據是當地司法制度長期以來被評為不健全, 而審判不公的情況更並非少有). The tutor didn't make any correction here, indicating I was right. I wanted to stress in this sentence the fact that miscarriage of justice (不公平審判) has occured time and again on the mainland, supporting my view that death penalty should be abolished there. That is why I used 'not uncommon' instead of writing 'miscarriages of justice are common'.
In fact, double negatives are also sometimes used in the Chinese language. Probably you have heard of something like：造成數十萬人在七一上街並非無因 (強調 ‘是有其原因的’)；‘並非無因’ is a form of double negative, much the same as 'not without reasons' in English.
Note: Try to make use of the double negative in your writing, but don't overuse it. When it is suitable, use this technique two or three times in say an essay of a few hundred words. Too much of it may make your writing a bit of a cliche.

Should be acceptable.

The reason why they use double negative might be because they'd like to emphasize on the commonness of the peer pressure. The term "not uncommon" does not directly imply "common", because they give quite different effects.