"...as soft opt-in does not considered as explicit consent under GDPR, it is not an acceptable practice. Soft opt-in is a form of temporary consent given by individuals while collecting their email details. Regardless how much individuals engage with your marketing communications, consent must be asked in explicit language. If the individual didn’t say “yes”, it means “no”." That's from Malijet.com regarding the Double-Optin discussion.
It doesn't matter if it's only one or thousands being from the E.U., you need to double-optin them - no matter what! Be aware of the fact that a lot of law firms watch out for different e-mail newsletters just to find out if you double-optin E.U. citizens or not. The punishments are up to 20 million € or 4% of your yearly worldwide generated income. There are already some cases where companies got sued because of GDPR violations. Be really careful with that stuff and take the safe route for the double-optin.