In a parallel plate capacitor of capacitance C, a metal sheet is inserted between the plates, parallel to them. The thickness of the sheet is half of the separation between the plates. The capacitance now becomes

2C

Before the metal sheet is inserted, $C =\frac{\epsilon_0A}{d}$

 After the sheet is inserted, the system is equivalent to two capacitors in series, each of capacitance $C' =\frac{\epsilon_0A}{d/4}$

The equivalent capacity is now 2C.