This is my workflow.
1. Single subscription form with e-mail and button, to be layed out in header and sidebar.
2. Form submission does not start subscription, but instead, it leads to subscription page.
3. Subscription page has 2 text fields: e-mail, automatically filled with value inserted in step 1; and e-mail confirmation, asking for user to repeat the e-mail as a verification (equality validation required). Then, a privacy check-box and submission button that, this time, actually starts subscription process in Newsletter plugin.