In the case you're Importing Tasks that already exist in eLogii and are duplicates, there are a number of options as to how to manage these Tasks during the CSV Import.
First, navigate to Configuration ⇒ Task CSV Import, and you can select one of the following options:
Create New Tasks
Block Import
Update Existing Tasks
For options (2) and (3) - Tasks are recognized as duplicates, either by their:
Task UID (generated by eLogii during Task creation)
Reference (if UID is checked and present in the CSV, Task will be matched only by UID, Reference will be ignored)
Click on Save once you have selected the most appropriate option for your operations.