So we all know that every fundamental force has a particles that mediates the force – strong nuclear force has the gluon, electromagnetism has photons, gravity has the hypothesised graviton and Weak force has W and Z bosons.

But why are there two different gauge bosons for the same force? Isn’t one boson enough to mediate a force? Does the independent existence of the two signify something?

The particles that mediate a force are defined when the field is quantized in QFT and has a symmetry group associated with it.

The generators of that group manifest as the quantum of that field.

The EM field has U(1) symmetry and has only one generator- the photon

The weak field has SU(2) as the symmetry and thus has 3 generators. These are W+,W- and Z0

But since this symmetry is broken, these particles are distinct and have different properties.

In case of strong field, it has SU(3) symmetry and is unbroken thus giving it 8 generators( there are 8 types of gluons) but they are all symmetric so nobody makes a distinction between them.

Study QFT to get more knowledge on this