When you enable "Prohibit duplicate values" option for multiple fields, duplicate check will be performed for the fields one by one. You cannot check for duplicate values across multiple fields.

For your reference, you can check for duplicate values across multiple fields using other features of Kintone.

Method 1: Check duplicate records using the Graphs feature

In this method, we use "Table" chart type in Graphs to find duplicate records.
For details on how to configure them, refer to the following page:
An error message "The unique field already contains duplicated values" appears. How do I check which records contain duplicate values?