You seem to be correct. I have been given the address with two 'n' by a shop that is in the Rhein-Neckar-Zentrum. When I saw that it was written with one 'n' here, I tried to look it up and ended up with both forms in different places (sometimes next to each other). As the writing with to 'n' is the most prominent of the usages from the shops in the place (and I didn't have time to drive there to look myself) I assumed (wrongly) that the two 'n' form is the correct one - as the people there could look themselves.

I'm sorry, I was wrong.