Here is my read.
SST was in your CR, and you DVd them. They did not respond. They did not have to.
Now a new CA named ACI has sent you a collection notice.
Apparently, ACI bought the debt from SST.
ACI does not have to post to your CR before they can contact you to collect the debt.
Yes, it is a bit confusing, but you should DV ACI within the 30-day period set forth in their dunning notice. The prior DV to SST is apparently not an issue now.
The post by SST can remain until such time as ACI posts to your CR. You can only have one CA on your CR at one time.
If ACI posts, then SST must delete.
Respond to the most recent dunning notice within 30-days, and DV each. If ACI now turns it over to another CA, that new CA must provide their own new dunning notice. They are hoping you dont DV within 30-days of their dunning notice.
Deal with credit repoting issues if and when they occur. I dont see them as of yet.