Hi ETconsultant,
1) After importing the file, D365 will parse the data from the file. The file import was successful, and the data in the file may not be parsed successfully.
If the data does not meet the business requirements, the parsing will fail. Not only duplicate data, but also other business needs.
Go to Setting > Data Management > Imports, check the resolution of the imported file.


As shown in the image, the file has been imported successfully, but not all data is successfully parsed.
The data of the first file is all parsed successfully. (Successes = Total Processed, Errors = 0)
The data in the sixth file was not fully parsed successfully. (Successes <Total Processed, Errors >0)

Perhaps the file you imported, the number of successfully parsed data is 0. So there is no new data in the list.
If so, then you have a data format error in the file. You need to check to find this error. It should be that all data has this error.
2) I don't recommend using "allow duplicate records" as this may disrupt business requirements.
The effect of allowing duplicate records is: if you already have the same data before, you are allowed to import this data again.
So if you don't have this record before, you are allowed to import it. Allowing duplicate records does not affect the import of new data.
Parsing also fails when there are conflicting records in your file. For example, two contact records for the same phone number. So allowing duplicate records does not help solve duplicate problems within the file.
3) Full Name = First Name + " " + Last Name. This is automatically stitched.
So the Full Name you enter must meet the requirement of this formula.
(A space in the middle is automatically added, you don't need to add a space at the end of First Name or at the beginning of Last Name. But when you manually enter the Full Name, you need to add a space.)
Perhaps it is because the Full Name format error caused the file data to be parsed completely failed.
Hope this helps.
Best Regards,
Lu Hao