Search: no warning about already having a Jetpack Search subscription during checkout

Steps to reproduce the issue

1. Go to https://jetpack.com/upgrade/search/
2. Enter a Jetpack site address that already has Jetpack Search plan

What I expected

If I enter a site address and that site already has Jetpack Search, I should be warned before the checkout.

What happened instead

The regular checkout page is displayed, telling me that I can buy Jetpack Search for £5.

Screenshots