Hi Stephen,
GoogleBot can't crawl links with a gclid, so they won't end up in the index and won't be flagged as duplicate pages.
AdWords does not work this way. The script that produces the ads contains no links--there's nothing for GoogleBot to discover without running the script. which it does not do. I am not certain that Google search produces the ads the same way, but what would be the point of GoogleBot crawling Google SERPs? It doesn't do that.
I do include canonical tag in the header of all our pages, which is one of Google's suggestions for dealing with this. I suppose that could be why I don't have an issue with this.
*If you don't believe this to be true, look through your logs and try to find one record of GoogleBot crawling an AdWords link with a valid gclid.
Hope this helps.
Thanks