  We show that because of the milder GIM suppression, the four-quark dipenguin
operator contributes to the short distance piece of the D^0-\bar D^0 mixing
amplitude at the same order of magnitude as the box diagram. In addition, we
put an upper bound \Delta m_D^{LDR} \leq 5 \times \Delta m_D^{box} on a
long-distance resonant contribution induced by the penguin mixing.
