Realtime should not have duplicate channels

[grdsdev]

Bug report

• I confirm this is a bug with Supabase, not with my own application.
• I confirm I have searched the Docs, GitHub Discussions, and Discord.

Describe the bug

Following up on the discussion on: supabase-community/supabase-kt#831 (comment)

Realtime client should not allow to have duplicate channels, when calling realtime.channel('my-topic') twice, it should return the same channel instead of pushing a new one to the channels array.