I'll look in to this one for you Moschusbock. We are able to see all the activites on an account in text form for the past few months, so it'll take me a while to parse through, but I'll get back to you.
In the mean time, duplicates are seen to not happen, but they will appear to be duplicated when you open the illusion list in the same session after having opened a Bogen-Box and found an illusion. You'll see it twice - one instance of the illusion being the "old system" type illusion, and one being the "new system" type - that is to say, one that takes up inventory space and one that is part of a master list of illusions that you own. Relogging is required to tidy up the "old system" style illusion from your inventory, freeing up precious space. We look in to a lot of these reported duplicates and in each instance, there hasn't been a dupe (since we fixed 'actual' dupes some months ago).