SlackAdapter.connect() overwrote self._handler, self._app, and
self._socket_mode_task without closing the prior AsyncSocketModeHandler
first. If connect() was called a second time on the same adapter (e.g.
during a gateway restart or in-process reconnect attempt), the old Socket
Mode websocket stayed alive. Both the old and new connections received
every Slack event and dispatched it twice — producing double responses
with different wording, the same bug that affected DiscordAdapter (#18187,
fixed in #18758).
Fix: add a close-before-reassign guard at the start of the connection
setup path, mirroring the guard DiscordAdapter.connect() already has.
When self._handler is None (fresh adapter, first connect()) the block is
a harmless no-op. Scoped to the handler/app fields only — no behavior
change for any path that does not call connect() twice.
Fixes#18980
6c1322b997