Google AdWords campaigns usually have a lot of keywords in multiple Ad Groups and in different matchtypes. Identifying duplicate keywords is extremely hard especially if you are not automating your AdWords Campaigns. Common misconception among advertisers is that by having duplicate keywords, your ad has a chance of showing up twice for a the same search. This is not true at all. Only one Ad per domain can enter a Google Auction and if you have duplicate keywords, They will compete against each other and will increase the CPC unnecessarily. This increase in CPC will drive up your costs which could have been low if not for duplicate keywords.
If you were using AdNabu to manage your AdWords Campaigns, We would have never allowed duplicate keywords to be added to your account. If a keyword is repeated twice in the account, it may or may not be a duplicate keyword, keywords of different matchtypes should be treated as separate keywords as the searches they can target is vastly different. Keywords with same matchtype and text are immediately identified by AdNabu and are not allowed to be created. We also identify duplicate keywords which might not be so obvious. For eg: a phrase match adwords and broad match adwords target the same set of keywords. Two broad matches in different orders for eg : adwords software and software adwords are one and the same.
There are however some special cases where keywords might repeat in your account. These are not considered duplicate keywords because they would not compete against each other. If the same keyword is in two different campaigns targeting different locations, they are non competitive and not duplicate keywords. If for some reason, you have multiple domains in your account and the keywords are repeated across domains, it is again not considered a duplicate keyword. Lastly if keywords are repeated in different types of campaign like text ads in search, call only, display or app install campaigns, they are again excluded from being treated as duplicate keywords.