How to determine if there are duplicate pages and find them?

Question

Search Engine Optimization

Actually a subject. On the Internet, very strange dances are offered and some kind of dances with tambourines.

Answer 1

1) download
2) compare
usually scripts for this

Answer 2

Good afternoon.
Use the sitereport.ru service. There is a "duplicate content" section.

Answer 3

Netpeak Spider
Trial I think is enough.