Yes, duplicates can exist across different People lists, but not within the same list.
In the All People list, duplicate entries are automatically merged and displayed as a single lead, so you won’t see them repeated there.
Allowing duplicates across different lists gives you more flexibility in organizing and customizing your People lists to suit specific needs or campaigns.
Currently, when importing leads from a CSV file, existing leads will not be duplicated. They will only be updated if the imported data contains new or additional information.