Remove duplication of Dns in config merging.

Fixes #4714
Docker-DCO-1.1-Signed-off-by: Alexandr Morozov <lk4d4math@gmail.com> (github: LK4D4)
Merge pull request #4775 from LK4D4/remove_duplicate_dns_#4714

Remove duplication of Dns in config merging.