trigger duplicatePhoneOnCandidate on Candidate__c(before insert ,before update) 
{
    Map<String, Candidate__c> candidateMap = new Map<String, Candidate__c>();
    
    Set<Id> setId = new Set<ID>();
    for(Candidate__c candidates: System.Trigger.new)
    {
        setId.add(candidates.Id);
        
        if( (candidates.Phone__c != null && System.Trigger.isInsert ) || ( System.Trigger.isUpdate  && candidates.Phone__c != System.Trigger.oldMap.get(candidates.Id).Phone__c ) )
        {
            if(candidateMap.containsKey(candidates.Phone__c))
            {
                candidates.Phone__c.addError('Phone Already exists');
            }
            else
            {
                candidateMap.put(candidates.Phone__c, candidates) ;   
            }
        }

    }  
    
    // Using a single database query, find all the Candidate in
    // the database that have the same Phone address as any
    // of the Candidate being inserted or updated.
    for ( Candidate__c can : [SELECT Phone__c FROM Candidate__c WHERE Phone__c IN :candidateMap.KeySet() and Id not in :setId ] ) 
    {
        Candidate__c newCan = candidateMap.get(can.Phone__c);
        newCan.Phone__c.addError('Candidate record have already this pone number.');
    }
}

Trigger duplicatePhoneOnCandidate on Candidate__c(before insert, before update) 
{
    Set<ID> setId = new Set<ID>();
    set<String> setEmail = new Set<String>();
    set<String> setPhone = new Set<String>();
    
    for (Candidate__c cand : Trigger.new) 
    {
        if (cand.Email__c != null && cand.Phone__c !=null) 
        {
            setId.add(cand.id);
            setEmail.add(cand.Email__c );
            setPhone.add(cand.Phone__c );
            
        }
    }
    
    if(setId.size() > 0 )
    {
        List<Candidate__c> lstOldCand  = [select email__c,phone__c,id from Candidate__c where email__c in :setEmail and phone__c in :setPhone and Id not in :setId ];
        Map<String ,Candidate__c> MapStringWiseCont = new Map<String,Candidate__c>();
        
        for(Candidate__c cand : lstOldCand  )
        {
            String key = cand.Email__c +'_'+cand.Phone__c;
            MapStringWiseCont.put(key,cand);
        }

        for (Candidate__c cand : Trigger.new) 
        {
            if (cand.Email__c != null && cand.Phone__c !=null) 
            {
                String key = cand.Email__c +'_'+cand.Phone__c;
                 if(MapStringWiseCont.containsKey(key))
                 {
                    Candidate__c contOld = MapStringWiseCont.get(key);
                    String errorMessage = 'Duplicate contact found! Record ID is ' + contOld.Id;
                    cand.addError(errorMessage);
                 }
            }
        }    
    }
}