Trigger preventDuplication on Lead(before insert, before update) {
Set<string> numSet = new Set <string>() ;
If (trigger.IsBefore) {
If(trigger. Is Insert || trigger.IsUpdate){
For(lead led: trigger.new){
numSet.add(led.Phone__c);
}
}
}
List<Lead> lstled = [select Id, Phone__c from Lead where Phone__c in :numSet];
For(Lead led2 : trigger.new){
If(lstled.size()>0){
led2.Phone__c.adderror('duplicate Number found');
}
}
Please Mark It As Best Answer If It Helps Thank You!
>> https://www.syedtaha.com/salesforce-apex-triggers/salesforce-apex-trigger-to-prevent-duplicate-contacts-by-email-or-phone-number/897/
The above link has a similar use case that you can try checking out.
Let me know if it helps you and close your query by marking it as solved so that it can help others in the future.
Thanks.
Try Below Trigger Please Mark It As Best Answer If It Helps
Thank You!