  We observe that the three-gluon form factor of the chiral part of the
stress-tensor multiplet in planar $\mathcal{N}=4$ super-Yang-Mills theory is
dual to the six-gluon MHV amplitude on its parity-preserving surface. Up to a
simple variable substitution, the map between these two quantities is given by
the antipode operation defined on polylogarithms (as part of their Hopf algebra
structure), which acts at symbol level by reversing the order of letters in
each term. We provide evidence for this duality through seven loops.
