When adding a new session to the internal session cache, it is examined whether a session with the same session id already exists. In this case it is assumed that both sessions are identical. If the same session is stored in a different SSL_SESSION object, The old session is removed and replaced by the new session. If the session is actually identical (the SSL_SESSION object is identical),
SSL_CTX_add_session() is a no-op, and the return value is 0.If a server SSL_CTX is configured with the SSL_SESS_CACHE_NO_INTERNAL_STORE flag then the internal cache will not be populated automatically by new sessions negotiated by the SSL/TLS implementation, even though the internal cache will be searched automatically for session-resume requests (the latter can be surpressed by SSL_SESS_CACHE_NO_INTERNAL_LOOKUP). So the application can use SSL_CTX_add_session() directly to have full control over the sessions that can be resumed if desired.