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You would be right if this molecule would be rigid. In that case one of the signals from the methyl group should arise from H1, and the second signal - with twice the intensity - should arise from H2 and H3.
However, the methyl protons in acetealdehyde show at room temperature only a single, shared signal. That is, the methyl protons have the same electronic environment and have the same chemical shift.

Try to come up with an explanation for the observation and compare your answer with the official solution.
(Hint: Is the shown structure the only possible conformation for acetealdehyde?)