Irrtum Justitia dont do courts. Their business model is to scare poor suckers with legalese. I would ignore them as long as they aren't sending registered letters. After that I would tell them I'd payed and threaten to go after them for harassment.
Just to clarify here. Irrrum injustitia is a debt collection company. What they do is buy bad debt off companies. Say you owe company x 2000 chf, but company x believes they have next to no chance of making you pay. Rather than write off the entire sum, they sell the debt to Irrtum Injustitia for say 200 chf. That has lessened their loss, and the rest is written off there and then. Irtum Injustitia then goes to you and goes through the motions of collecting that money off you, and it slaps on a little extra charge hoping you'll be gullible enough. Irrtum knows that only a fraction of the people it threatens pay, but it also makes enough on the people who do that they're still making a good profit.
Now what you've done is payed x that money after they had written off the debt and sold it to Irrtum. This puts them in a predicament. They thus try and get Irrtum to give them back the 200 chf, but Irrtum doesn't like victory being snatched from its jaws so is still going after you to recover at least something. Or in fact if company x is unlucky they may be unable to reverse the trade and x is having to pass those 2000 on to Irrtum and Irrtum still have the gall to go after you for a little extra.