In Peskin Schroeder, section 10.2, the contributions to M2(p2) of order λ2 are calculated. They respective Feynman diagrams are given:
why is this diagram NOT included?
Answer
Just before their statement that the first non-zero corrections to the ϕ4-propagator are those you indicated, they compute the one-loop corrections, using on-shell renormalization conditions, and find out that they are zero:
It seems that for the O(λ2) (two-loop) corrections, one should indeed consider not only the diagram you pointed out but another one with a counterterm insertion, which cancels it due to the above one-loop result in the considered scheme. Schematically:
Where the fourth diagram corresponds to the one you drew and the fifth term is the O(λ2) contribution with the counterterm insertion.
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