I just noticed however, that I deducted my prescription costs as a medical expense. Since what one pays to one's pharmacy via prescriptions is how the deductible is collected, then if I claimed 'a separate deductible,' I'd be claiming this amount twice. So I think if I've already claimed my prescriptions, then I've covered this.

You are correct - if you have already claimed these amounts as prescriptions, do not list the same expense.