How to escape a Duplicated Content Penalty.

Selected answers from the Dumb SEO Questions Facebook & G+ community.

• 
  Peter Cornish: How to escape a Duplicated Content Penalty.
  A site's main SEO issue was a domain parked on top, presumably resulting in a dup content penalty. ;
  
  Have changed the parked domain to a 301 redirect (should it just be removed it altogether?) and replaced most of the site's text a few weeks ago but still no significant organic performance change
  
  Have we done enough to clear the dup content penalty ?
  What lead time is it likely to be before we see organic performance change ? ;
• 
  Richard Hearne: If you do have a penalty you're likely looking in the wrong place. Duplicate content is generally ignored rather than penalised, and simply parking a domain (which sounds more like forwarding it to your main site) shouldn't cause any penalty.
  
  This would be supported by your mention of updating the content without change in ranking.
  
  When did this penalty occur, what's your site domain, how old is the site?