In general it is best not to duplicate keywords which is why Google have a tool within the AdWords Editor to find them. However sometimes you will find yourself duplicating keywords because one campaign targets the content network and the other campaign targets the search network as an example.
If you are targeting the same place but in two separate campaigns or ad groups, only one ad will be displayed for that particular search. You will never have two of your ads displayed for a given search query.
That is called double serving and it is against the rules.
I'd like to keep them seperate as I want to test the use of banner ads ONLY in the content network and not have my text ads appearing in the content network.