My online payment failed once then I made a payment, and it went through. Why do I show two charges?

When you attempt to make a payment, a pre-authorization is done.
When it fails, it is denied but a pre-auth may still show on your account with your
institution but is removed "automatically"--usually within 2-3 business days, depending on your institution.
Since technology is so fast, people who view online accounts can see pre-auths, etc., which normally would never be shown on a printed statement; say, via mail. It is only our quick view now of our online accounts that we see this before it is actually removed.

Your account will only be charged once, and you will see only one charge once this pre-auth is removed.