Error 451 - noninteger values in a longitudinal dataset

Hi everyone,
I'm currently working on my masters thesis and my subject is about bilateral development aid from donor to recipient for the years 1980-2019. It is a longitudinal dataset and in total I have 44.400 observations (30 donors - 37 recipients - 40 years) - so for every donor I've got 37 recipients and each recipient 40 years. I imported the dataset from Excel.

When I enter the command xtset recipient_id year, I receive the error "repeated time values within panel" with the explanation that my variable contains noninteger values. I already checked for duplicates so that's not the issue. The original type of the year variable is "int" and the format is "%10.0g".

How can I solve this problem so that I can indicate my identifiers?

Thank you so much in advance!