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Your LimeSurvey version: 6.5.6
Own server or LimeSurvey hosting: LimeSurvey
Survey theme/template:
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When importing CPDB entries from a csv-file, there is an option to avoid duplicates.
When importing the same file again, all entries have duplicates although the option was selected.
Problem is reproducable.

Can anyone confirm this?

I have seen a bug 18919.

The current bug report tracking this problem is here: bugs.limesurvey.org/view.php?id=19558

Problem is still present in 6.10.2, did I miss something?