Is there an actual rule that says a person that created a post is not allowed to delete it as long as it is marked as a duplicate?
If the answer is yes then you should change the system, so the delete button under the post should be deactivated after the post has been marked as a duplicate.
If not, then I can assume that my post was un-deleted twice by a moderator without a legitimate reason?
SFF Stack Exchange has a very clear community policy that questions should not be deleted simply because they're duplicates. Jason Baker's answer to the linked meta discussion provides lengthy arguments for why this would be a bad idea, but the TL;DR is that leaving duplicate questions undeleted increases searchability and it's always possible they may be reopened.
In your particular case, the question of where Dementors came from is a valid question about the Harry Potter universe which many people are likely to find interesting. And while the answer does exist on a previous question (hence the closure of yours as a duplicate), that question is entitled Did Ekrizdis create the Dementors?, which is a much less searchable title. Far more people will have heard of Dementors, which appear prominently in the HP books and films, than the wizard Ekrizdis, who as far as I know is only ever mentioned in a single Pottermore article. From the point of view of providing information to the maximum possible number of people searching for this question on the interent, your title Where do Dementors come from? is much more useful.
Also please remember that closure is not necessarily a permanent state. You believe your question is not a duplicate; if enough other community members agree with you, it may be reopened. Deleting your post is a way to ensure that nobody will ever see it again to upvote it or vote to reopen it.
Like it says in the small print at the bottom of every Stack Exchange page:
user contributions licensed under cc by-sa 3.0 with attribution required
In this sense, you don't "own" the contents of your post, so you shouldn't just delete the entire question and replace it by "ffs........", as you did twice. There's no network-wide rule or policy on deletion of duplicates, but SE is entitled to keep a post around on the site if it's useful content.
You can, however, request dissociation of your username and credentials from the question, so that it will show as having been posted by "anon" rather than "hedgehog", and will disappear from the list of your posts and activity. This can only be done by Stack Exchange employees, so if you'd like your post to be dissociated, you can either use the contact us link at the bottom of every page, or raise a moderator flag on the post so that we can pass on your request to the appropriate people.
(Note that I don't know much more about the SE licensing system than I've written here. If you want to know more details, please look around or ask on main meta.)
Rand's answer is an excellent explanation of SE policy and why your specific question should remain undeleted (it increases searchability, and you can edit it to possibly re-open it). Additionally, it's worth noting that Law.SE has a question about getting one's own questions/answers deleted which notes that
Users grant StackExchange a licence:
You agree that all Subscriber Content that You contribute to the Network is perpetually and irrevocably licensed to Stack Exchange under the Creative Commons Attribution Share Alike license.
As the moderator who responded, I will explain what happened from my point of view. I initially responded to an auto-generated flag for too many comments. As I started pruning the comments I noticed that you were quite upset and were abusive to other users. Consequently, my first comment to you was:
You are clearly upset, but ranting about it is not going to solve anything. Please consider stepping away from the computer and taking a break.
Your first response to me (which you later deleted) indicated that you intended to "rage" and "troll":
yeah when i get misstreated and missjugded i rage a lot.... cause i know i am trying to explain somethign that i know its correct but none listens.... so troll mode on, yeah why not...
I therefore reminded you of the "be nice" rule (since you were abusive to other users) and told you that trolling would not be tolerated:
@hedgehog You are not being mistreated -- other users simply judged your question to be a duplicate and voted to close it, as is their right. You are not allowed to rage at other users -- you have to be nice. Trolling will not be tolerated.
Your response to that was to vandalize your question and delete it (after you had already vandalized it once before). I undeleted it, and you re-deleted it. I undeleted it once again and commented with links from Meta.SE explaining SE policy on deleting duplicates and vandalized (even self-vandalized) posts:
For the last time, do not delete duplicate questions. Also, do not vandalize your posts. You've now deleted it twice and vandalized it twice.
To summarize your behavior:
Because of this behavior on your part, I warned you that you would be suspended if you continued such behavior. I also kept a close eye on the question throughout the day in case you came back to continue your behavior (that's how I noticed you kept deleting it).
Under more normal circumstances I would not have objected so strongly to the deletion of your question. I would have recommended you keep it undeleted per SE policy and for searchability, but I would not have undeleted it if you'd insisted. However, as I indicated in the comments, you appeared to be deleting your question in order to make good on your threat to engage in "troll mode".
As I said in my first comment to you, take a break and calm down. If you still think the question should be deleted after you've calmed down I won't undelete it. @Rand locked the post for a week, so you have until then to calm down and make a rational decision to delete it.
