I have a client who changed his domain to incorporated his prime keyword. The transition from the old domain to the new domain has not proceeded as smoothly and swiftly as the client hoped. He is now proposing pointing both the old domain and the new domain at the same website. I have told him that I think that would be considered "dupilcate content" and could harm his ranking, but he thinks he has seen other companies do it successfully. I'm interested in opinions on this issue. Please feel free to comment. PS. I did advise him to do a 301 redirect from the old domain to the new.
If both domains were displaying the same content then it would be duplicate. As you suggested, he should 301 redirect the old domain to the new one.
I have one domain which is under two different domains and nothing wrong happend, but maybe I'm just luck (this site is not so important to me, so I can take it as an experiment . But ofcourse, every case is different.