Why does oxygen not like to be a double anion?
The addition of an electron to any negatively charged ion will need to overcome the repulsive force of the negative ion. This is generally less favorable than adding an electron to a neutral atom, and the electron affinity will be positive. Even though the additional electron will complete the octet, as in the case of an  ion where the addition of electron will complete the  subshell, the electron affinity is a positive value.