Duplicate content with multilang pages?

I have a setup with two languages and i am using the slug for the second language pages.

Project page example…
DE: domain.de/projekte
EN: domain.de/en/projects

But domain.de/en/projekte is also a valid URL.
Should it not redirect to domain.de/en/projects, or even return a 404?
Should i worry about duplicate content?

Thanks in advance!

I don’t think so because these URLs will not appear anywhere so bots will not index them because they only follow existing links.