I have a CSV file containing duplicates contacts. We would like to identify those duplicates in our DE to exclude them in an email send. Do you know any solution for this issue?
Deduplication should be done before importing to Marketing Cloud. There is no reason why you wouldn't be able to use Excel functions to find and remove duplicate entries from your CSV file.
In any case, if you really prefer doing this the hard way - after importing your records to a Data Extension, you can use SQL queries to find the duplicates and then post unique records to a new Data Extension.
Another option is to decide on an attribute you would use as
PrimaryKey in your Data Extension configuration, as this is required to be unique. For an example, you could use Email as the unique value if you don't have any ID's for the contacts. This way you would be able to import only rows with unique Email address values.