Google Adwords Add or Update Keywords

We are trying to use Adwords API to add keyword to an existing adgroup. We'd like to implement it so it ignores keywords that already exist and inserts only the ones that do not exist in the adgroup.

Currently, API inserts all keywords which results in duplicate keywords. We are using code found on this page:
http://adwords-api-dotnet-samples.googlecode.com/svn-history/r25/trunk/src/AddKeyword.cs
Please refer us to the documentation that we should look at or suggest what we should do. We'd appreciate any help.

// Add keyword if there are no policy violations.
        if (errors == null) {
          Criterion[] criteria = service.addCriteria(
            new Criterion[] {keyword});

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