best way to merge domains

Google indexing site under two names

[peterpum]

I have a site that is in the index with about 12000 pages. It has been like this for over a year now.

The site is listed under two domains:

Dom1: <companyname>.com
Dom2: <keyword1>.co.uk

Both domains are on the same host/same IP and show exactly the same content, still goggle shows results from both domains in the search-results.

I'm thinking about merging the two by redirecting every request to Dom1 to the same URL on Dom2 via a 301 header because i fear a duplicate content penalty in the future.

Is there a better way to do this? Should I do it at all?

Thx