I was long puzzled by messages such as these:
[SUHPL] 05:09:09 INFO: Connected to already connected device 7ASDDJG-W4YEKEY-OTAORH7-REYEKSP-4MIBIHS-IQYEQI4-CLRJARE-DBJDLQ4 (existing: 127.0.0.1:43470-127.0.0.1:22001/tcp-client/TLS1.2-TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384 new: 127.0.0.1:22000-127.0.0.1:55082/tcp-server/TLS1.2-TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384)
The solution is not obvious. I have a simple configuration between two devices, although I’m doing this through an ssh tunnel with manual configuration (not discovery). Maybe this is why the unusual condition happened to me.
In the config.xml on each device, there was a device entry for both devices, which seemed somehow logical to me but is actually incorrect. There should be device entries only for the remote device(s).
To fix it, you must stop syncthing and remove the local device - both in the section and the entire section. I did this on both sides, and then the mysterious messages stopped coming.