In Google Webmaster Tools, I noticed that my front page was indexed two times:
I know that I could fix this with the use of link type
canonical, but I wonder: Where does this parameter come from?
There are various sites that have pages indexed with this very parameter/value: https://duckduckgo.com/?q=chocaid%3D397.
I looked for similarities between these sites. but couldn't find a conclusive one: It's often the front page, but not in every case. Some are NSFW, but not all. When one domains' URL has this parameter, often other subdomains of the same domain have it, too.