I'm currently going through the interaction part of Tong's lecture notes on quantum field theory, and I'm a bit confused about how many Feynman diagrams I need to draw in order to compute the amplitude of various interaction terms. Tong presents Feynman diagrams through a series of examples. For instance for nucleon-nucleon scattering Tong draws two diagrams, with the labels for the outgoing momentum flipped on one diagram with respect to the other. However for meson-meson scattering there is a single diagram. Why don't we need a second diagram in the meson-meson case with the labels of the outgoing momentum flipped on the second diagram? I don't understand how to see this without using Dyson's formula.