I have a problem with the canonical-urls at my website www.bauredakteur.de.

For example:
www.bauredakteur.de/blog/32-innenausbau/laminat-kaufen-11-tipps-vom-experten.html will look like
www.bauredakteur.de/blog/32-innenausbau/laminat-kaufen-11-tipps-vom-experten/321-laminat-kaufen-11-tipps-vom-experten.html

The alias is double in the Url and google indexing this wrong Url - very bad.

How can I fix this.

You need to disable Joomla!'s canonical urls for K2.

www.robertwent.com/blog/joomla/87-how-to-remove-joomla-s-canonical-tag-without-hacking-the-core

You have to use an extension which supports K2 to add canonical links to your K2 items.