Nearly 2 weeks later, I can report that the bug of receiving multiple notifications for the same post has been fixed.
What Still Is Not Fixed: You still receive email notifications for posts only in those threads that you yourself originated. That is probably not what the forum software author intended, as the Email me replies to this post check-box is available for you (and activated by default!) even when you post a reply to a thread not originated by you.
Related Bug: The check-box just mentioned is only available for you once you submit your post and then re-open it via the Edit link. (The same bug occurs for the Quote button and Preview Post, likewise not available as you are composing your post for the first time. I'm opening a separate bug report thread for that important issue, as it's not directly related to email notifications.)
Clarification Desirable: The check-box is currently described as Email me replies to this post, but a clearer, more appropriate solution might be a three-way radio button allowing the user to choose one of the following three options: Email me replies to this post / Email me replies to this thread / Do not notify me of replies.
Functionality Extension Desirable: The same 3 options should ideally be made available even to forum users who haven't contributed to a particular forum thread yet; perhaps a link at the bottom of each thread, titled Subscribe to This Thread, might activate email notifications for that particular thread for any user who clicks that link. An analogous amenity would be to place a Subscribe to This Forum link at the bottom of each sub-forum, allowing users to click the link and receive email notifications for each newly created thread in that sub-forum. To save bandwidth on GWT's mail servers, email notifications would not necessarily have to contain the text itself of the post they point to; they would simply contain a clickable link to the new post submitted to the thread / sub-forum. Also, it's not necessary for email notifications to be generated in the HTML format; switch to plain-text, and you've instantly reduced the email notifications bandwidth on GWT's mail servers by half at least. To save even further bandwidth, only one notification might be sent for a new reply to a thread, and no new notifications for that thread would be sent to a user before that user actually revisits the thread. (All of the above is standard functionality for leading forum software packages such as vBulletin and phpBB.)
Related Bug: You are currently also emailed copies of your own posts. (But only in threads you yourself originated, see the main bug detailed above.) You either shouldn't be notified of your own posts, or there should be a separate check-box titled Email me a copy of this post in addition to the proposed three-way radio button Email me replies to this post / Email me replies to this thread / Do not notify me of replies, so that user expectations are set correctly.
Thanks a lot to WGT for fixing these email notification bugs / obliging these feature requests in future! Naturally, the importance of these bugs / feature requests is secondary compared to the game itself.