I am getting this error when I try to run the code below. Please help.

Error:

System.Web.Services.Protocols.SoapHeaderException was unhandled
  Message="A duplicate value was specified for field 'Id' in object 'Opportunity', duplicate value '00650000008ov0VAAQ' prior value '00650000008ov0VAAQ'"

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Code:

sObject[] myOppties;

myOppties = sfdc.retrieve("Id, AccountId", "Opportunity", new String[] { "00650000008ov0VAAQ", "00650000008mpS4AAI", "00650000008m20QAAQ" });

for (int i = 0; i < myOppties.Length;i++)

{

if (myOppties[i].Any[0].InnerText == "00650000008m20QAAQ")

{

myOppties[i].Any[1].InnerText = "0015000000IRt4nAAD";

}

System.Console.WriteLine(myOppties[i].Any[0].InnerText + " " + myOppties[i].Any[1].InnerText);

}

SaveResult[] mySaveResult = sfdc.update(myOppties);