Hello, @lukasbrandau
Default WordPress, also Mailchimp services are not allow your to use the same email multiple times for subscription.
You should install our Newsletters plugin on your website and create there Subscribers list.
Then you should open required popup and go to Subscribe settings. There you can choose our Newsletters plugin Subscribers list to collect subscribers: http://prntscr.com/js1bgg – here is explanation screenshot.
It will allow you to use the same email address as many times as you want.
Let us know, if you have additional questions about this solution.