If you've read one of Stephen King's novels, you know his tendency to just give up at the ending.

It got me thinking of the good number of my clients who have brought me to clean up or actually get things working for them after their big migration or upgrade project.  They paid premium rates to only be left shell-shocked, resentful, and lost. 

Campaign Member records are unique in the Salesforce world in that a Contact can only have one Campaign Member record per Campaign. Process Builder is very literal.  If you set up Process Builder to create a new record, but that Contact is already a member of that Campaign, Process Builder errors out.