I always include every entity that has claimed have the right to collect the debt. Sometimes that means three or four collectors in addition to the original creditor. I also notify courts where lawsuits have been filed, and include the case number in the address to assist the court clerks in locating the file so it can be flagged by the court as a bankruptcy matter.
Why wouldn't you list them? Strictly speaking, are you required to, no. But should you, yes...if you want to avoid the continued hassle of that firm calling you and then you sending the notice of bankruptcy yourself later.