Hmmm. Are you sure that's not Lindemann's Lambics? (I know, old post, but maybe he's still here)
Well I'm not sure about the Lindemann's products; Im not really familiar with them. Other than the Faro, which I think by definition is back sweetened?
My understanding of Duchesse being back sweetened was based entirely on word-of-mouth and of course various posts on the interwebs. However, your question made me go back and try to find a definitive reference to Duchesse being artificially sweetened. I found a number of posts in various forums alluding to Duchesse being sweetened but nothing definitive. I did a quick flip through Wild Brews and it mentions that Duchesse is blended and pasteurized/filtered, but I didnt find a reference to it specifically, as being back sweetened.
Then after some further trolling of the interwebs, I came across this post in the BA forum
Duchesse de Bourgogne - Belgium - BeerAdvocate
To summarize, its an email from Karl Verhaeghe at Brouweri Verhaeghe in which he states:
we did not, are not or have not the intention to use aspartame, nor saccharin or anything of these kind of products in our Duchesse de Bourgogne as well as any other of our other beers.
So unless something has changed, or this is just a bogus email (I kinda doubt it), Duchesse is in fact
NOT back sweetened with any artificial sweetener and I stand corrected.