Welcome to the community @VELOVATE 

There are a few considerations with consent:

• Klaviyo’s T&Cs. For Klaviyo SMS you must always have explicit consent. For marketing emails, Klaviyo prefers explicit consent. More info here: https://help.klaviyo.com/hc/en-us/articles/360002057172
• Complying with country regulations (e.g. GDPR), For example, in the UK, we must have explicit consent for marketing emails plus an unsubscribe link. in some countries, it is OK to not have explicit consent - so-called implied consent - so long as there is an unsubscribe link. Transactional emails do not require consent or an unsubscribe (see below). I cannot advise you on the regulations specific to Germany, maybe others on here can.
• Best practice for email engagement and sender reputation. If a contact has not provided explicit consent, they are less likely to engage with your emails and/or flag them as spam - either will potentially damage your sender reputation. 
• Transactional emails. If the emails to your customers are transactional e.g relating to an order they have placed, Klaviyo will allow you to send those in a flow without opt-in and without an unsubscribe link. However, you need to approach Klaviyo support to gain approval for these transactional flows before they can start sending.

I don’t believe double opt-in is part of the European GDPR requirements. However, it prevents lists from getting invalid email submissions and spammers, so it is worth considering.

I hope that helps

Regards

Andy