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The parasitic diodes are inherent to the FET's structure. The point where the diodes meet is the body of the FET, and this node must be biased somewhere for proper operation. Normally the body node is connected directly to the source of the FET, which in a CMOS circuit will be either GND or Vcc. This is the case for the N channel FET in your schematic. One of its parasitic diodes is effectively short circuited, and does nothing. But for the P channel FET here they put an extra diode in series to Vcc in order to prevent excessive current flow from the output to Vcc.
Parasitic diodes should not be counted on for any current
circulating / shunting. They may (depending on construction)
inject substrate current causing latchup above some weakly
specified threshold, or may just corrupt analog circuitry.
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