You were a victim of a drive-by closing...at least as far as my vote is concerned. I definitely won't remember something from back in April at this point.
I upvoted your question just now and nominated it for re-opening.
The question is decent enough in that the Canonical doesn't specifically cite that only a single SPF record can be used...otherwise I would still agree with the duplicate reasoning.
Almost ALL of the questions regarding SPF records here come from devops setting up a web server with SMTP and wondering how to set one up that really have no idea what an SPF record even is. It was only a matter of time before most of the regulars here started seeing the word SPF in a question and immediately thinking "close...dupe". Your question just happened to be one of those that IMO is decent enough to stay but that we simply didn't read carefully enough.