Is it redundant to use the "boost" module if varnish is used?
No, with the correct configuration, Boost and Varnish can work together. The combination is said to speed up your site even more than just Boost or Varnish (I haven't tried it myself yet).

See Will Boost conflict with Varnish and Use Varnish for cache storage instead of file system. If you're visiting DrupalCon London, check out the Boosted Varnish talk.