function Contains(
    AHandle: TIdStackSocketHandle
): boolean; override;

boolean - True when the socket handle already exists in the list.

Contains is an overridden boolean function that implements the method used to determine if the socket handle in AHandle is already stored in the socket list. 

Contains searches the ArrayList that implements storage for the list. The return value for the method is the result from Contains method in the ArrayList that implements storage for the list.