A common misconception in Google AdWords is that it is fine to use the same keywords in different campaigns. Although AdWords allows the use of the same keyword in multiple campaigns or ad groups, it will cause negative effects. The first issue this causes is your ads will be competing with each other and Google will only display one of your ads for the given keyword at a time. Therefore you lose opportunities for visibility. The second issue is that because you are competing with your own keywords and ads you are driving the cost per click up on those keywords. Google discusses this in their Improve Your Keywords for the Search Network
article along with how to find duplicate keywords.
Removing duplicate keywords is an easy way to reduce unnecessary spend. It is strongly suggested that you structure campaigns in a logical order by grouping keywords together. This will assist you in avoiding duplicate keywords in various campaigns and ad groups. Also be sure you are using the best match type for a given keyword, especially with long tail keywords.