Here is the whole code if it helps any...

global with sharing class CloneOpportunity 
{
    public static final String PRICEBOOK_NAME = 'CTM Price Book';
    
    WebService static String clone(String oppId)
    {
        final String ACTIVITY_HISTORY_SUBJECT = 'Products Missing from ' + PRICEBOOK_NAME;
        final String COMMONS = 'The following products are not associated with the ' + PRICEBOOK_NAME;
        
        // Clone opportunity
        String pricebook2Id = [select Id from Pricebook2 where Name = :PRICEBOOK_NAME limit 1].Id;
        RecordType recordType = [select Id from RecordType where SobjectType = 'Opportunity' and Name = 'Renewal Opportunity' limit 1];
        String oppQuery = getOpportunityQuery(oppId);
        Opportunity sourceOpp = (Opportunity)Database.query(oppQuery);
        Opportunity newOpportunity = sourceOpp.clone(false, true, false, false);
        newOpportunity.RecordTypeId = recordType.Id;
        newOpportunity.Pricebook2Id = pricebook2Id;
        newOpportunity.Contract_Number__c = '';
        newOpportunity.Record_Type__c = 'Quote';
        newOpportunity.StageName = 'Needs Analysis';
        newOpportunity.Thank_You_Card_Email__c = '';
        newOpportunity.Thank_You_Card_Contact__c = null;
        newOpportunity.Purchase_Order__c = '0' ;
        newOpportunity.GreatPlainsCustomerID__c ='';
        if(newOpportunity.Renewal_Cycle_Follow_Up_Date__c != null)
        {
            newOpportunity.Renewal_Cycle_Follow_Up_Date__c = newOpportunity.Renewal_Cycle_Follow_Up_Date__c.addYears(1);
        }
        if(newOpportunity.Contract_Start_Date__c != null)
        {
            newOpportunity.Contract_Start_Date__c = newOpportunity.Contract_Start_Date__c.addYears(1);
        }
        if(newOpportunity.Contract_End_Date__c != null)
        {
            newOpportunity.Contract_End_Date__c = newOpportunity.Contract_End_Date__c.addYears(1);
        }
        if(newOpportunity.Distribution_Dates_Start_Date__c != null)
        {
            newOpportunity.Distribution_Dates_Start_Date__c = newOpportunity.Distribution_Dates_Start_Date__c.addYears(1);
        }
        if(newOpportunity.Distribution_Dates_End_Date__c != null)
        {
            newOpportunity.Distribution_Dates_End_Date__c = newOpportunity.Distribution_Dates_End_Date__c.addYears(1);
        }
        if(newOpportunity.Bill_Dates_Start_Date__c != null)
        {
            newOpportunity.Bill_Dates_Start_Date__c = newOpportunity.Bill_Dates_Start_Date__c.addYears(1);
        }
        if(newOpportunity.Bill_Dates_End_Date__c != null)
        {
            newOpportunity.Bill_Dates_End_Date__c = newOpportunity.Bill_Dates_End_Date__c.addYears(1);
        }
        if(newOpportunity.CloseDate != null)
        {
            newOpportunity.CloseDate = newOpportunity.CloseDate.addYears(1);
        }
        insert newOpportunity;
        
        // Clone opportunity products
        Set<String> productIds = getProductIdsFromLineItem(oppId);
        Map<String, String> ctmProductIds = new Map<String, String>();
        for(PricebookEntry entry : [select Id, Product2Id from PricebookEntry where Pricebook2Id = :pricebook2Id and Product2Id in :productIds and CurrencyIsoCode = :newOpportunity.CurrencyIsoCode])
        {
            ctmProductIds.put(entry.Product2Id, entry.Id);
        }
        
        String lineItemQuery = getOpportunityLineItemQuery(oppId);
        OpportunityLineItem[] sourceLineItems = (List<OpportunityLineItem>)Database.query(lineItemQuery);
        List<OpportunityLineItem> newLineItems = new List<OpportunityLineItem>();
        List<String> lineItemsInActivity = new List<String>();
        String productNames = '';
        
        for(OpportunityLineItem item : sourceLineItems)
        {
            if(ctmProductIds.keySet().contains(item.PricebookEntry.Product2Id))
            {
                OpportunityLineItem newLineItem = item.clone(false, true, false, false);
                newLineItem.OpportunityId = newOpportunity.Id;
                newLineItem.PricebookEntryId = ctmProductIds.get(item.PricebookEntry.Product2Id);
                if(newLineItem.Billing_Start_Date__c != null)
                {
                    newLineItem.Billing_Start_Date__c = newLineItem.Billing_Start_Date__c.addYears(1);
                }
                if(newLineItem.Billing_End_Date__c != null)
                {
                    newLineItem.Billing_End_Date__c = newLineItem.Billing_End_Date__c.addYears(1);
                }
                if(newLineItem.Contract_Start_Date__c != null)
                {
                    newLineItem.Contract_Start_Date__c = newLineItem.Contract_Start_Date__c.addYears(1);
                }
                if(newLineItem.Contract_End_Date__c != null)
                {
                    newLineItem.Contract_End_Date__c = newLineItem.Contract_End_Date__c.addYears(1);
                }
                newLineItems.add(newLineItem);
            }
            else
            {
                String productName = ' • ' + item.PricebookEntry.Product2.Name + '\r\n';
                lineItemsInActivity.add(productName);
                productNames += productName;
            }
        }
        if(newLineItems.size() > 0)
        {
            insert newLineItems;
        }
        
        if(lineItemsInActivity.size() > 0)
        {
            Task task = new Task();
            task.WhatId = newOpportunity.Id;
            task.Subject = ACTIVITY_HISTORY_SUBJECT;
            task.ActivityDate = Date.today().addDays(-1);
            task.Status = 'Completed';
            task.Type= 'SF Message';
            task.Description = COMMONS + ':\r\n';
            for(String lineItemName : lineItemsInActivity)
            {
                task.Description += lineItemName;
            }
            insert task;
        }
        return productNames + '^o^' + newOpportunity.Id;
    }
    
    /**
     * Get the opportunity query.
     */
    private static String getOpportunityQuery(String oppId)
    {
        String result = 'select ';
        for(String fieldApiName : Schema.SObjectType.Opportunity.fields.getMap().keySet())
        {
            result += fieldApiName + ',';
        }
        result = result.substring(0, result.length() - 1) + ' from Opportunity where Id = \'' + oppId + '\'';
        return result;
    }
    
    /**
     * Get the opportunity product query.
     */
    private static String getOpportunityLineItemQuery(String oppId)
    {
        String result = 'select ';
        for(String fieldApiName : Schema.SObjectType.OpportunityLineItem.fields.getMap().keySet())
        {
            result += fieldApiName + ',';
        }
        result = result.replace('totalprice,', '');
        result = result + 'PricebookEntry.Product2Id, PricebookEntry.Product2.Name from OpportunityLineItem where OpportunityId = \'' + oppId + '\'';
        return result;
    }
    /**
     * Get the product IDs in opportunity.
     */
    private static Set<String> getProductIdsFromLineItem(String oppId)
    {
        Set<String> result = new Set<String>();
        for(OpportunityLineItem item : [select PricebookEntry.Product2Id from OpportunityLineItem where OpportunityId = :oppId])
        {
            result.add(item.PricebookEntry.Product2Id);
        }
        return result;
    }
    
    @isTest
    public static void testCloneOpportunity()
    {
        OpportunityLineItem line = [select OpportunityId from OpportunityLineItem limit 1];
        CloneOpportunity.clone(line.OpportunityId);
    }
}