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?

Method 2: Customize your app with API and JavaScript and perform the duplicate check against multiple fields (Only available for the kintone Standard license plan)

In this method, we use a character concatenation plug-in, which is provided as a sample.
Concatenate strings of the fields you want to perform the duplicate check, and enable the Prohibit duplicate values option for the field that shows the concatenated string.
Character concatenation plug-in

However, if you import data from an Excel/CSV file, you cannot perform the duplicate check because the data will not be reflected automatically to the field that shows the concatenated string. After you configure the character concatenation plug-in, you need to add or edit records on the screen.

If you have any questions on sample plug-ins, use the contact form for API to ask for assistance.
Reference: Who should I contact about API?