Hi Christina,
To ensure our leads do not receive content they already downloaded or were previously emailed, we
use programs in our engagement streams. In our Send Email smart campaign, we use filters to ensure no one who has downloaded the asset (such as member of SFDC campaign not in, member of program not in, not was sent email, not visited web page, etc).
For webinars, you can add the asset and edit the availability of it in the stream. More information here: https://community.marketo.com/MarketoArticle?id=kA050000000LAWOCA4
You can prioritize the webinar by adding it to the top of the stream as well: https://community.marketo.com/MarketoArticle?id=kA050000000LAWJCA4