When a delegate creates a new `Revision`, their signature is implicitly added to the revision.
Prevent the delegate from an accept verdict on two active, sibling revisions by:
1. Checking that the `state` is `Active`, and then checking `has_active_sibling_accept`
2. If there is an existing verdict, then it the verdict from the new revision is stripped.
The creation of the `Revision` must still happen, as opposed to throwing an
error, in case of existing histories.
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