Amazon does do a lot to detect bribed reviews automatically. However, in the long run, this may be an arms race that Amazon will lose. One issue with detecting fakes with identical phrases is that many real reviewers will take phrases from the product description or from other peoples' reviews either consciously or unconsciously. This also matches up with a common bribed review technique of giving people canned phras…
This is exactly the problem with the "consumer protection" racket--companies like Consumer Affairs will curate a negative presentation of your company or product, then call you up offering a $2,500/month subscription which basically allows you to remove old negative reviews and prevent new ones from appearing on their site.