you just need to use a meaningful name for the method by using NAME field available in OperationContract attribute. So you need to give a name to both or one of the methods to avoid this problem. As you can see in the below code that I did in control IService.cs interface.
[OperationContract(Name = "GetIntegerData")]
string GetData(int value);
[OperationContract(Name = "GetStringData")]
string GetData(string value);