"A CoAP client MUST set the Request-Tag option in the case of different concurrent blockwise request operations to the same resource. If the CoAP client is running a blockwise request operation without the Request-Tag, and wants to concurrently start another blockwise request operations to the same resource without cancelling the first, then the CoAP client MUST set the Request-Tag."
These sentences are quite similar. I think the second sentence is sufficient and the first can be removed.
"A CoAP client MUST set the Request-Tag option in the case of different concurrent blockwise request operations to the same resource. If the CoAP client is running a blockwise request operation without the Request-Tag, and wants to concurrently start another blockwise request operations to the same resource without cancelling the first, then the CoAP client MUST set the Request-Tag."
These sentences are quite similar. I think the second sentence is sufficient and the first can be removed.