Agreed. Trying to locate identical problems, whether in the solutions manual or by indexing solved problems, will never work. First of all, the PE exam will contain completely different problems than those in the various study guides. They are not simply the same problems with different numbers. Second of all, even if there were many repeat problems, you simply wouldn't have enough time to locate them and reapply them to the given problem. You may encounter a couple problems out of 80 that are very similar to ones in the study guides, but simply not enough to justify that as a valid means of passing the test. Spend the extra time you would have used indexing problems and just keep working more problems and reviewing the material. I put together a binder with a handfull of solved problems and never even looked at it. Thankfully I didn't spend much time putting that together. Many people put together a binder of literally a thousand solved problems. Much time is spent doing this and I just don't think the payback is very good.
The introduction section of MERM goes over the various study methods used in preparation for the exam. They specifically mention the method we are talking about here. They indicate that it is almost never successful. Take the solutions manual with you just in case you get lucky on 1 or 2 problems, but don't expect many problems to be identical.
Good luck.