Right there on the site is a restriction in place to installing more than one copy for the same user -- which is likely why your second copy does not work as it should.
Site does not state it is an avast certified reseller. It appears to be an ecommerce-based reseller.
See attached below for Terms and Conditions posted at the website.
Copy may even be limited only to a 180-day trial license, no standard user license may be available at the end of the trial period.
I'd steer clear of this offer as it seems not be trustworthy and may even have hidden unknown code embedded within it.
Remember, an antivirus program is one of the most powerful stand-alone programs one can install on a system, so it has have extreme integrity and be proficient and trustworthy at the same time.
@@@@@
Always get your files, free or paid, at the vendor's site only.