Change:
13.4. Initialization keys are intended to be used only once and then <del> deleted.  Reuse of init keys is not believed to be inherently insecure dhreuse, although it can complicate protocol analyses. </del> <ins> deleted.  Reuse of init keys can lead to replay attacks. </ins> 14.

Discussion:
This has been reported unclear multiple times. I strongly prefer forbidding reuse altogether. A validation mechanism is needed for the formal security proof, it can be implemented according to an expiration mechanism (see Architecture - 4.2. Delivery Service Compromise)
We need to allow Last Resort keys. Not sure what you mean by "replay attacks"
I rewrote it in favor of using "SHOULD NOT reuse", which is true, except in case of last resort to avoid DOS, which we are willing to allow. Normal meaning... If I remember correctly, there is a cross channel forwarding attack (replay) possible on the first message in Signal when reusing prekeys (the last resort key) because it lacks some channel binding but I would have to check. I am working to make sure we don't suffer the same defect but I am documenting it in the mean time.
I can live with this for now; we can refine later.
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