How to avoid 'the same signature error' in interface?

I am modeling domain-interfaces for n-tier application and preparing interface ICustomerRepository. Inside I have a plenty of signatures and two of them faced with 'same signature' error.

    Customer GetCustomer(string guid, CustomerInformationLevel informationLevel
        = CustomerInformationLevel.AccountInformation);

    Customer GetCustomer(string firstName, string lastName, string middleName,
        CustomerInformationLevel informationLevel
            = CustomerInformationLevel.AccountInformation);

    Customer GetCustomer(string email, CustomerInformationLevel informationLevel
        = CustomerInformationLevel.AccountInformation);

And signatures with string 'guid' and 'email' conflicts. I know why it happens but don't know how to avoid it in context of the right architecture and design principles.