Claws Mail, version 3.18.0, compiled from source. openSUSE 15.2 Linux asrock 5.3.18-lp152.81-preempt #1 SMP PREEMPT Mon Jul 5 23:37:31 UTC 2021 (4022e93) x86_64 x86_64 "Delete Duplicated Messages" used to delete all duplicates and keep only one copy of the set (as it should). "Delete Duplicated Messages" now deletes ALL copies of duplicated messages (all duplicated messages of a set are permanently deleted including the original). Best, Lyle
I'm afraid I'm not reproducing what you describe here, one of the duplicated messages is left as expected, others are trashed.
Same for me: works as intended without error.
I've reverted to claws version 3.17.8git94 which didn't have this issue. I lost several important emails because of this bug after upgrading to 3.18.0. Perhaps it's related to openSUSE 15.2??? Historically, I've never had a problem with deleting duplicates in "all folders". I do it often as I get duplicates regularly from where I am a member of several lists that send the identical message to all members of each list.
The only changes between 3.17.8git94 and 3.18.0 were translation updates and document text file edits. What you describe is not possible.