Atlas shows duplicates shown when a bridge changes to a publicly listed node.
If a node is running as a bridge with PublishServiceDescriptor bridge then changes to a regular node without resetting it's fingerprint, that node will have 2 identical entries forever.
I have reproduced here: https://atlas.torproject.org/#search/30AFA68C0F9B30C213F19B6CE57F7D8CA4B77A25
Trac:
Username: entry