How did you delete it?
There is some discussion about using the delete command manually (e.g. from commandline through the web UI): How does retention work?
The restore points exists both in the local database and on the remote store as a dlist, so the delete command has to remove both.
It’s possible to do operations with --no-local-db if there is an inconsistency, this is what Direct restore from backup files ... does on the restore page.
In either case, I’ll look into if there’s an issue on Github or if one should be created.
EDIT: I found List of backup timestamps contains duplicates · Issue #2857 · duplicati/duplicati · GitHub so it’s noted as a milestone, but it hasn’t be fixed yet.