why my code in page_load loading twice ?

Hi,

Why my function loading twice. i have written a function and calling that function in page_load() event.

E.G :

Private double Sumoftwono()
{
sum= a + b;

return sum;
}

in page_load()
{
if (!Page.IsPostBack)
{
Sumoftwono(); //this loading twice
}
}

I am using a master page, web-controls for header and footer.

Any ideas?

Thank you