Better.cs

public class SomeClass
{
    private readonly List<Customer> _customers = new List<Customer>();

    public void AddCustomerForModelIfValid(CustomerModel modelOfCustomer)
    {
        if (IsModelValidForAdding(modelOfCustomer))
            AddCustomerForModel(modelOfCustomer);
    }

    private bool IsModelValidForAdding(CustomerModel modelOfCustomer)
    {
        return modelOfCustomer.IsActive && !_customers.Any(c => c.Name == modelOfCustomer.Name);
    }

    private void AddCustomerForModel(CustomerModel modelOfCustomer)
    {
        var customerBasedOnModel = new Customer()
        {
            Name = modelOfCustomer.Name
        };
        _customers.Add(customerBasedOnModel);
    }
}