Here is what happens and what don't:
SPA sends INVITE to UserB (lets say) 101
UserB sends OK with CSeq 101
UserB sends re-Invite with CSeq (lets say) 1
SPA sends OK with CSeq 1
UserB sends another re-INVITE with CSeq 2
SPA sends an OK with CSeq 1.
The remote sequence number doesn't change after the second re-INVITE.
See rfc3261 section 12.