AWeber's defaults for optins are as follows:
1. Imported Addresses - Double Optin Required
2. Emailing - Double Optin Required
3. Web Form Submissions - Double Optin is not Required, but highly recommended
MemberGate has set up an email parser with AWeber so any new members joining your site are added to your AWeber list. These new signups require a double optin.
As a rule of thumb, there is an option to not have a double optin if a member specifically puts their information into an AWeber form. If a member's information gets onto an AWeber list by any other means, a double optin is required.
To prevent spammers or others from playing with your Tip of the Week form by signing up friends or worse, the Tip of the Week should be setup as a double optin.