So there is mod called C2ME and it advertises own module named c2me:ext_render_distance_v1.
The problem is the logline Joined server with c2me:ext_render_distance_v1 support is printed twice like this:

[14:49:17] [Render thread/INFO]: Joined server with c2me:ext_render_distance_v1 support
[14:49:17] [Server thread/INFO]: a[local:E:da49f53e] logged in with entity id 19 at (-113.26158840148204, 105.24918707874468, 492.6856965643458)
[14:49:17] [Server thread/INFO]: a joined the game
[14:49:17] [Render thread/INFO]: Started 10 worker threads
[14:49:17] [Render thread/INFO]: Joined server with c2me:ext_render_distance_v1 support

The mod author said that the subject to blame is Fabric API, so im asking to do investigation on your side.
Fabric Loader is 0.16 and API is 0.100.8+1.21

We obviously don't log that line, so we can't do anything with that information.

Update: checked C2ME's code, the log comes from C2SPlayChannelEvents.REGISTER event. The event is explicitly documented as "may be invoked at any time after login and up to disconnect".

Works as intended. Users should implement checks themselves if they do not want duplicate calls.