Hello Eelco,
If you call addCriteria() and specify a Criterion object that already
exists for a given ad group (for instance, a Keyword who text and type
are both identical to an existing Keyword in the ad group) then the
call will still succeed. If you specify a different value for, say,
maxCpc in the newly added Criterion, then the existing keyword will
have its maxCpc value updated to the newly submitted value.
The addCriteria() call costs 25 units per Criterion included in the
request, regardless of whether the Criterion previously existed.
Cheers,
-Jeffrey Posnick, AdWords API Team