[IPsec] Multiple IPsec proposals, same SPI?

[Dan McDonald <[e-mail]>]

Consider the example in section 3.3 of IKEv2bis, which I've edited for
brevity:

   SA Payload
      |
      +--- Proposal #1 ( Proto ID = ESP(3), SPI size = 4,
      |     |            7 transforms,      SPI = 0x052357bb )

	(either way for ESN, three CBC ciphers, two hashes)

      +--- Proposal #2 ( Proto ID = ESP(3), SPI size = 4,
            |            4 transforms,      SPI = 0x35a1d6f2 )
	(either way for ESN, two GCM ciphers)

the example shows two distinct SPI values.

Is it *required* that the SPI values be different?  For example, PF_KEY has
SADB_GETSPI which merely reserves an inbound SPI value, without ANY other
properties attached.  IN theory, given the above example, I could instead
issue:

   SA Payload
      |
      +--- Proposal #1 ( Proto ID = ESP(3), SPI size = 4,
      |     |            7 transforms,      SPI = 0x052357bb )

	(either way for ESN, three CBC ciphers, two hashes)

      +--- Proposal #2 ( Proto ID = ESP(3), SPI size = 4,
            |            4 transforms,      SPI = 0x052357bb )
	(either way for ESN, two GCM ciphers)

since I merely did a GETSPI which reserved 0x052357bb.

Thanks,
Dan