Two properties of a photon that I have considered in trying to answer this myself :
Photons are electrically neutral, so there is no need for "anti-photons" to preserve conservation of charge.
Take a massive particle, say a spin-1/2 electron moving away from us with its spin measured to be +1/2 in the direction of it's motion. Now, because the electron cannot move at the speed of light, then we can move to a frame of reference ahead of the electron, we look back and see the same electron now having a spin of -1/2.
Now try the same trick with a photon with a helicity measured to be 1. We can't move to a frame of reference faster than the speed of light, so for all observers this helicity must be invariant.
My question is, does the invariant helicity property, if true, contribute to the concept of a photon and an "anti-photon" being the same entity?
If not, why not, and are there any other properties unique to photons that I have not considered?