Depends on D19893. Ref T13222. See PHI873. A challenge is "answered" if you provide a valid response. A challenge is "completed" if we let you through the MFA check and do whatever actual action the check is protecting.
If you only have one MFA factor, challenges will be "completed" immediately after they are "answered". However, if you have two or more factors, it's possible to "answer" one or more prompts, but fewer than all of the prompts, and end up with "answered" challenges that are not "completed".
In the future, it may also be possible to answer all the challenges but then have an error occur before they are marked "completed" (for example, a unique key collision in the transaction code). For now, nothing interesting happens between "answered" and "completed". This would take the form of the caller explicitly providing flags like "wait to mark the challenges as completed until I do something" and "okay, mark the challenges as completed now".
This change prevents all token reuse, even on the same workflow. Future changes will let the answered challenges "stick" to the client form so you don't have to re-answer challenges for a short period of time if you hit a unique key collision.