Question: Nothing in my nearly two years of using SO has given me the impression that SO seeks to monopolise the Q&A forum world to the extent that questions asked here may not be asked elsewhere.

I have posted a few questions here for which I have received no replies in the time that it normally takes to get a reply. They all deal with ASP.NET Dynamic Data, which in my experience is not a very well known or popular subject on SO. So, I went and asked them on Microsoft's ASP.NET forums, in the Dynamic Data section.

It has taken user XIII less time than it took me to duplicate them, to mark them all as duplicates on both SO and the ASP.NET forum. I find this behaviour akin to policing both forums for the wrong reasons, and would like to know whether I am allowed to repeat questions in forums unrelated to Stack Exchange or not, and if not, why not?

Answer: 
You are allowed to duplicate them. Within the sites there is obvious control over duplication within the same site, but there is no rule or anything stopping you from posting the question elsewhere as well. However, it would be recommended not to link between the two, but rather keep them separated altogether. You can in a comment maybe provide a link to the same question on another site, but keep in mind the idea is that people searching in future will find the question here.

Within the SO sites, duplication is discourage because of the canonical question ideal. One question, one complete and up to date answer. Two important things to keep in mind, asking a question on SO does not guarantee an answer, and secondly, bounties and updating questions can help them get noticed and answered faster.

To answer your question, if a user is giving you a hard time about this on the ASP.Net forums, it's not related to the SE sites. It is the user itself that feels it should not be allowed.

All questions and answer are posted require CC-Wiki attribution, however, this is only relevant when reposting and reprinting the complete questions and answers, if you do have a link to this question on other sites, pointing back here, then it is 100% acceptable to do this. However if the other sites discourages this practice, your also welcome to remove the links, since it's not complete reproductions of the original SO/SE content.
Did your post to the ASP.NET forum state you'd posted it on stackoverflow already and link to the SO question? Can you expand on "in the time that it normally takes"? 

Personally I find it pretty annoying when people duplicate post questions *without indicating they've done so*. (Equally both questions should standalone; a question saying "Can you answer this SO question?" would likely not be well received, whereas a full post saying "I've also posted this on SO but it's been a week and no one there seems to know" should not be problematical.)

If we're talking a week or more then maybe it's okay, if you mean hours I can see why people would get annoyed - it's annoying to read the same question in multiple locations, doubly so to spend time composing a reply in one place only to later find it was already answered perfectly well by someone else already.