Only check each mirror once for DNS-RR. 🔼

There's a race condition, which I think is coz now I check deb.devuan.org and it's RR IPs as well as checking each mirror for DNS-RR ability.  Resulting in those DNS-RR mirrors being checked twice.

reported=2020-08-22 18:40:52

reporter=onefang

priority=immediate

category=Bug

severity=block