There is one more extension which is worth checking out - https://www.magentocommerce.com/magento-connect/geoip-redirect-by-amasty.html, it should work ok and redirect your site visitors automatically according to their coutry IP.
As for SEO, don't forget to set up a hreflang tag to eliminate any problems which may be caused by duplicate content.
This might help you ,
Better to make a custom module and override the default Store Manager model
Hi there, I thought this would be a solution too, however installing this violates Google's Merchants rules - Fine if you're not using Google shopping through! :0)
The problem you faced is actually a very common one. It usually appears because of the absence of the alternate tags on those pages.
(For those who haven't heard about it yet, alternate tags are tags used to connect pages that are different variations of one page in several languages. They are used to avoid content duplication issues and 404 pages instead of different language content problems)
To solve this issue you need to set alternate tags for your English and German pages marking them as different variations of the same page. This is a technical task that requires some technical skills.
However, for those who don't have any, there is the Magento 2 Hreflang Extension that allows setting alternate tags directly from the admin with no technical skills at all.