> I sent registration instructions to all of them within 8 hours, but only 6 people clicked the link in that email.

I think they drew the wrong conclusion from their data. I don't think it was the double signup that was the problem, it was the delay between signing up and getting the confirmation email.

I think the real lesson here is that confirmation emails need to be short (addressed in the article) and quick.

I know that if I don't get the confirmation email within about a minute, I give up.