The Small Copper (Lycaena phlaeas) looks similar to the Large Copper (Lycaena dispar) and Sooty Copper (Lycaena tityrus).
However, the underside of the Large Copper's hind wing is light blue and features an orange band.

The male Sooty Copper is distinguished from the Large and Small Copper by having both brown forewings and hindwings.
The female Sooty Copper is best distinguished by the forewing and hindwing having a sharp orange edge with clear black spots.
In the female Small Copper this orange edge is not nicely filled with sharp black spots and sometimes even very unclear.

1st photo: In my yard, Hilversum, The Netherlands, 20 August 2018
2nd & 3rd photo: Boekel area, The Netherlands, 6 July 2018
4th photo: Strabrechtse heide, Heeze, The Netherlands, 13 May 2012
5th photo: Dwingelderveld National Park, The Netherlands, 24 July 2012