Can we overload methods in WCF Service or Web Service?

Yes for a WCF Service use the Name property of OperationContractAttribute class
example:
[ServiceContract]
interface ddd
{
[OperationContract(Name = "one")]
int calc(int a,int b);

[OperationContract(Name = "two")]
double calc(double a,double b);
}

1)For a Web Service use the MessageName property of WebMethodAttribute class
2)Please comment the following line in the .cs file of the Web Service
//[WebServiceBinding(ConformsTo = WsiProfiles.BasicProfile1_1)]
[WebMethod]
public string HelloWorld(string a) {
return "Hello"+" "+a;
}
[WebMethod(MessageName="second")]
public string HelloWorld()
{
return "Hello second";
}