Understanding the Work Email Waterfall: Explaining Credits and Invalid Emails

When using a Work Email Waterfall, if the 1st email provider finds an email, but then it's found to not be valid...then it will run a search on the other email providers, right? But if each email provider keeps turning up the same invalid email 4-5 times in a row...am I charged 2 credits for each attempt at finding the same invalid email address? I'm trying to figure out how I burned through so many credits in just a few attempts, and without many valid emails to show for it. Thanks for your help in figuring this out!