If using RegisterClientScriptBlock than you can query if you allready register your client-side script.
Like in this example from msdn:

 // Form the script that is to be registered at client side.
      String scriptString = "<script language=JavaScript> function DoClick() {";
      scriptString += "myForm.show.value='Welcome to Microsoft .NET'}<";
      scriptString += "/";
      scriptString += "script>";

           if(!this.IsClientScriptBlockRegistered("clientScript"))
             this.RegisterClientScriptBlock("clientScript", scriptString);