Consolidating two different domains to point at same site, duplicate content penalty?

[Ron10]

I have two websites that are extremely similar and want to consolidate them into one website by pointing both domain names at one website.  is this going to cause any duplicate content penalties by having two different domain names pointing at the same site?  Both domains get traffic so i don't want to just discontinue one of the domains.

[MonicaOConnor]

If you are 301 redirecting these sites eventually the older URLs will no longer be indexed, leaving only the one site with it's unique content. You don't have to worry about duplicate content in this instance.