Hello Developers

i am new to the apex development, i have been given a task for creating Triggers to prevent duplicate records on multiple fields , i know this is can be achived through Duplicate Rules, but still i want to try with the Apex trigger

Assignedto__c (Lookup(User)

Quater_Year__c(Picklist)

Month__c(Picklist)

i want to create a Duplication rule on these 3 fields, 

i refered the below link, but i could not get the correct code

https://www.mstsolutions.com/technical/preventing-duplicate-records-based-on-multiple-fields-in-salesforce/