If you are running several ad groups or pay per click campaigns simultaneously and using the same keyword in those ad groups, you should seriously reconsider. It could be hurting your ad groups.
Yahoo! Search Marketing’s blog tells you why:
For example, if you want to test Advanced and Standard match types by running both concurrently in identical ad groups, your keywords will compete against each other and drive up your cost-per-click. We’ve also seen advertisers duplicate keywords with multiple landing pages. The duplicated keywords can result in a lower quality index score and a higher cost-per-click.
I can’t think of any better reasons than these to stop duplicating your keywords. Higher CPCs, lower quality scores, and duplicate keywords that get deleted. Wouldn’t want any of that to happen to me.
And Yahoo! isn’t the only search engine with these policies. You could run into the same problems at other pay per click providers like Google and MSN adCenter. To be most effective, run tight ad groups with specific ad content and landing pages that don’t overlap. There is no sense in competing against yourself.