It wouldn't matter anyway.
The online "checkout system" is dumb, but the actual Verizon system is not. Whenever it notices you have a duplicate feature on there, you'll immediately see it drop off under MyVerizon within 24 hours and you will not ever see a charge for the duplicate feature.
It's just an error in the checkout system...RELAX.
If you're that concerned with it, do the transaction over the phone or in the store.