Yes. It's not unusual.
Disc ID's are not as unique as they should, unfortunately. So, sometimes it happens that you try to contribute with a Disc ID that already exists.
The message from freedb tells you that there is already another disc with the same ID in the rock category. What you can do (and what people usually do) is to contribute it to another category. If there is none in that category with the same ID freedb will accept it. That's why there are so many CDs appearently in the wrong category at freedb. Because there was a discID collision on the right one.