Here in below example the trigger is on “Account” object, to check duplicate Account Names.

Apex Trigger:

trigger accountDuplicateCheck on Account (before Insert, before Update) {
    
    Map<String, Account> accMap = new Map<String, Account>();
    
    for (Account acc : System.Trigger.new) {
        
        //Make sure we don't treat account Name that isn't changing during an update as a duplicate.  
        if (System.Trigger.isInsert || (acc.Name != System.Trigger.oldMap.get(acc.Id).Name)) {
            
            //Make sure another new account isn't also a duplicate  
            if (accMap.containsKey(acc.Name)) {
                acc.Name.addError('Another account has the ' + acc.Name + ' same name.');
            } else {
                accMap.put(acc.Name, acc);
            }
        }
    }
    
    //Query to find all the Accounts in the database that have the same name as any of the Accounts being inserted or updated.  
    for (Account acc : [SELECT Name FROM Account
                        WHERE Name IN :accMap.KeySet()]) {
                            Account newAcc = accMap.get(acc.Name);
                            newAcc.Name.addError('An account with this name ' + acc.Name + ' already exists.');
                        }
}