I wrote that library originally for dl.google.com: https://go.dev/talks/2013/oscon-dl.slide#1

I then open sourced it in Jan 2013 in what was then named Camlistore (now Perkeep) in https://github.com/perkeep/perkeep/commit/6f9f0bdda9c9c1f147... d

And later I put it in https://pkg.go.dev/github.com/golang/groupcache/singleflight (groupcache was written for dl.google.com)

And a private copy in Go's net package in Jun 2013: https://github.com/golang/go/commit/61d3b2db6292581fc07a3767...

It later moved to golang.org/x/net, and later to the Go standard library (well, internal: https://pkg.go.dev/internal/singleflight)

We now even have a copy with generics in Tailscale's tree at https://pkg.go.dev/tailscale.com/util/singleflight

So many variants of that code :)