I’ve just experienced this issue right now and it is incredibly annoying.
I tried to post a question regarding a problem I’m currently having, but the title was the same as another post. I changed the title and then tried to re-submit, but was met with the “You can only post once every 90 minutes.” error. Now I am stuck waiting 90 minutes before I can post the issue I need help with.
A failed submission of a post should not count towards a user’s post limit, it should allow the user to make the corrections required and then submit. It is possible this is a bug that needs fixed, or it may be the intended design of the system, but either way I personally think this is a big flaw.