如果我在主题中通过 此处 的 API 创建帖子，第一次可以成功，但在 raw 内容更改之前，后续请求均无法成功。
Yes, you can’t create posts with the exact same content as another post (except if you’re an admin).
I have an admin api key? Any way to have the rate limits & permissions from that apply but still make the post as the user?
You will need to post as admin and then issue another API call to change the author.
Users will see the post being sent by the admin user then changing users though… that’s not ideal. Is their any better way?
Yes, if the message is identical then don’t send duplicate PMs. Post it as a pinned announcement.
If it’s only going to a subset of users create a group and category which identifies them if one doesn’t already exist.
I’m talking about messages in a topic. With thousands of posts in a topic, it’s conceivable that the same thing may have to be said twice by the same user, no?
I hope you’re not using a discussion software as a chat software 
In a discussion, repetition can be useful but only if you do it right. If you want to get your point accross, saying it over and over using the exact same words won’t work (and often has the opposite effect). You hve to put yourself in the shoes of your audience and adapt your words accordingly.