301 redirect alias domain to main domain, Will this hurt SE results?

Our alias domain shows up before our main domain in google results. 301

[tupots]

I've read a lot of forums on this topic but am still unclear about a couple of things. BTW, our site is hosted on an apache server.
We have a site that originally had 1 domain name. Google indexed the site. We changed the name of the company and bought a new domain name and kept the old domain name as an alias. Everything is with 1 hosting company, and both domains point to exactly the same content. I know how to make a 301 redirect so that all requests for the old domain name as well as the non-www version will resolve to the new domain name with the www.
My question is whether this is a good idea since in our google results the old domain is the one that shows up although google has indexed pages from both domains. I'm obviously worried about duplicate content. All 4 versions of the homepage (old domain name, new domain name, www & non-www of each) have a PR of 4. Any ideas?

[wmuser]

I remember it has been discussed many times,the best option is to search WW for similar threads

[John Carpenter]

301 redirect alias domain to main domain, Will this hurt SE results?

On the contrary. 301 redirect from all "alias" domains to the main domain is recommended to prevent SE "duplicate content penalty".

[tupots]

Thank You John,
I will implement the 301 because at this point I have nothing to lose. Our site is not even coming up for our company name anymore, possibly a dup. content issue, or google readjusting itself or something else. We'll see what happens...

[tupots]

after checking again, we are coming up for our company name but have lost significant ground for our keyword terms. this was before the redirect. Hopefully the 301 will help.