Question:  where a member starts a renewal at one level and then, for reasons at aren't clear, starts another renewal at a different level. They are then Renewal - Pending at the latest level, but have two outstanding invoices.

Response:  If you have the option set so that someone can change levels, then what is probably happening is that they are getting a renewal invoice for the 1st level and when they go in to renew, they are changing to a different level will generate another new invoice. However, if their existing membership level is "Pending" the option to change is not available in their profile. My suggestion would be to try this out yourself to see what the status is when you send out your renewal invoices. 

Resources:  https://gethelp.wildapricot.com/en/articles/162-renewing-membership-or-changing-levels?_ga=2.104720837.490028022.1628870612-1607974617151#level