rec: Fix the aggressive cache returning duplicated NSEC3 records

rec: Fix the aggressive cache returning duplicated NSEC3 records

No need to include the same record twice when it provides, at the same
time, a proof that the closest encloser exists and that the next closer
does not, and/or that the wildcard does not exist either.
This happens right away in a zone with a single record, like reported
by Matt Nordhoff, but it might happen in other cases as well.

(cherry picked from commit eca47c4dd85d2711ebc26767dfa78bdbce2c7f16)