Trigger code:-
-------------------
/* Write a trigger that will prevent a user from creating a lead that already existed as a Contact.
 * We'll use the lead/contacts email address to detect duplicates.
 * Whenever the lead is created or updated.
1. Lead has an email address
2. Try to find a matching contact based or email address.
3. If a match is found, give the user an error.
4. If a match is not found, do nothing.
== if email matched based on contact record in the lead record at the time of creating through an error
*/
trigger DuplicateContactsOnLead on Lead (before insert,before update) {
    for(lead l:Trigger.new){
        if(l.Email != Null){
            list<contact> con =[SELECT ID FROM Contact WHERE EMAIL=:L.Email];
            if(con !=null && con.size()>0){
               // string ErrorMsg ='Duplicate Value Found!!';
               // ErrorMsg +='RecordId is:-'+con[0].id;
                l.Email.addError('Duplicate Value Found!! Record Id Is:='+con[0].id);
             
            }
        }
    }
}
-----------------------------------------------------------------------------------------------------
Test  Class Code:-
--------------------------
@isTest
private class TestClass_DuplicateContactsOnLead {
   
    static testMethod void testme(){
        contact c = new contact(LastName='testing',Email='sreekanth@gmail.com');
        insert c;
        lead l = new lead(LastName='TestLead',Email='sreekanth@gmail.com',Company='wipro',Status='Open - Not Contacted');
        if(l.Email != Null){
            list<contact> con =[SELECT ID FROM Contact WHERE EMAIL=:L.Email];
            if(con !=null && con.size()>0){
                l.Email.addError('Duplicate Value Found!! Record Id Is:='+con[0].id);
        }else{
            insert l;
        }
        }
    }
}