Only extract rkt if it hasn't been. Only Fetch if it's missing. Verify SHA512 of the tarball before extracting.
Discovered while recreating pathological failure conditions on local vagrant cluster and recreating vagrant clients in a loop over night (read: overnight testing revealed that rkt-1.2.0.tar.gz.723 didn't vary in its contents compared with rkt-1.2.0.tar.gz)
