Like I said in my opening post, I used the "reset URL" in the SEO-tab -> no difference
Adding the reference -> no diverence
In the original product, the reference-field was blank, and the seo-link was correct
The moment I started to duplicate, it went wrong in every possible way
And... I can't save the product, after adding the refference.
I get the red banner at the top left: "unable to safe settings"'.