Two domains, one site

I have a client who has a site we have optimized. He has another domain that now points to the optimized site. I'm not sure what method he as used, but what is the correct method of doing so?

I believe it has caused our optimized site to drop in the rankings.

Thanks in advance.

 

301 permanent redirect

 

I agree. A mod-rewrite 301 permanent redirect is the best way to go.

There is VERY high likelihood Google will blacklist both domains otherwise.

 

Thanks, that is what I had surmised as well.