Unfortunately, I haven't come across anyone in the past few months with this kind of issue. It is better to call IRCC so that they resolve the testing duplication on their side and also send an email to the Niagara office. The test window most likely got invalidated when it was duplicated by the Windsor office.
The test link remain the same, irrespective of the office or when they send them so accessing from one or the other email will not make a difference. It is on the test server, which thinks that your test window is still not open so not letting you login. Hope one of the office resolves it for you in the test server.
Below is the test link, which is a common / generic link sent by all offices.
https://www.canada.ca/en/immigration-refugees-citizenship/services/canadian-citizenship/become-canadian-citizen/citizenship-test/online.html
This is the link to sign-in to the test server during the test window.
https://ct-tc.apps.cic.gc.ca/?lang=en