update item changes programatically on itemtype save event

Question

hi all,

i set property of item programmatically using following code  (C# code)

Innovator inn=this.getInnovator();
string InvID = this.getProperty("id");

if(InvID !="" && InvID != null)
{
//get invoice item
Item Invitem=inn.newItem("bal_invoice","get");
Invitem.setProperty("id",InvID);
Item ResInv=Invitem.apply();
int InvCnt=ResInv.getItemCount();
if(InvCnt!=0)
{
//get inoive item property
decimal transportAmt=0;
decimal cessAmt=0;
string transport=ResInv.getProperty("bal_transportation");
if(transport!=null)
{
transportAmt=Convert.ToDecimal(transport);
}
string cess=ResInv.getProperty("bal_cessamt");
if(cess!=null)
{
cessAmt=Convert.ToDecimal(cess);
}

//get invoice relation item
Item relInv=inn.newItem("Bal_Invoice2Parts","get");
relInv.setProperty("source_id",ResInv.getID());
Item Resrelinv=relInv.apply();
int relinvCnt=Resrelinv.getItemCount();
// return inn.newError(relinvCnt.ToString());
decimal AmtTotal=0;
decimal IgstTotal=0;
decimal CgstTotal=0;
decimal SgstTotal=0;
decimal tTotal=0;

for(int i=0;i<relinvCnt;i++)
{
Item iInv=Resrelinv.getItemByIndex(i);
//get invoice relation property
string amount=iInv.getProperty("bal_amount");
string igstamount=iInv.getProperty("bal_igstamt");
string cgstamount=iInv.getProperty("bal_cgstamt");
string sgstamount=iInv.getProperty("bal_sgstamt");
string totalamount=iInv.getProperty("bal_totalamt");

//get total of all relation property
if(amount!=null)
{
AmtTotal=Convert.ToDecimal(AmtTotal)+Convert.ToDecimal(amount);
}
if(igstamount!=null)
{
IgstTotal=Convert.ToDecimal(IgstTotal)+Convert.ToDecimal(igstamount);
}
if(cgstamount!=null)
{
CgstTotal=Convert.ToDecimal(CgstTotal)+Convert.ToDecimal(cgstamount);
}
if(sgstamount!=null)
{
SgstTotal=Convert.ToDecimal(SgstTotal)+Convert.ToDecimal(sgstamount);
}
if(totalamount!=null)
{
tTotal=Convert.ToDecimal(tTotal)+Convert.ToDecimal(totalamount);
}
}
decimal gtotal=Convert.ToDecimal(tTotal)+Convert.ToDecimal(transportAmt)+Convert.ToDecimal(cessAmt);
// return inn.newError(AmtTotal.ToString());

//set calculated total to invoice
this.setProperty("bal_taxablevalue",AmtTotal.ToString());
this.setProperty("bal_igstamount",IgstTotal.ToString());
this.setProperty("bal_cgstamount",CgstTotal.ToString());
this.setProperty("bal_sgstamount",SgstTotal.ToString());
this.setProperty("bal_totalamount",tTotal.ToString());
this.setProperty("bal_gross_total",gtotal.ToString());

i added this code on itemtype onAfterAdd and onAfterUpdate

but i have to click on save button twice to update item form property

to avoid this i adde code-

if(this.getAction()!="add")
{
this.apply("update");
}

but i get an error - item with same key alreay added

please help me