fix: make setup_logging idempotent to avoid duplicate log lines

setup_logging unconditionally addHandler'd on every call. Under the new
main.py entry the function was invoked twice for --loop mode (main.py and
service.run both called it), stacking two file + two console handlers so
every log line was written twice.

Clear any pre-existing root handlers before re-adding so repeated calls
always yield exactly one file handler + one console handler, regardless of
the caller. Preserves the standalone python -m sync.service entry point.

src/sync/logging_setup.py

@@ -18,12 +18,26 @@ def setup_logging(cfg_dict: dict | None):
     level = getattr(logging, (cfg_dict or {}).get("level", "INFO")) if cfg_dict else logging.INFO
     path = (cfg_dict or {}).get("path", "sync.log")
     os.makedirs(os.path.dirname(path) or ".", exist_ok=True)
+
+    # Idempotent: drop any handlers already attached to the root logger before
+    # re-adding. setup_logging can be called from more than one entry point
+    # (e.g. main.py and service.run), and the old code unconditionally
+    # addHandler'd each time, stacking duplicate handlers so every log line was
+    # written twice. Clearing first means repeated calls always yield exactly
+    # one file handler + one console handler, regardless of caller.
+    root = logging.getLogger()
+    for old in list(root.handlers):
+        root.removeHandler(old)
+        try:
+            old.close()
+        except Exception:
+            pass
+    root.setLevel(level)
+
     h = logging.handlers.RotatingFileHandler(
         path, maxBytes=10 * 1024 * 1024, backupCount=5, encoding="utf-8"
     )
     h.setFormatter(logging.Formatter("%(asctime)s %(levelname)s [%(name)s] %(message)s"))
-    root = logging.getLogger()
-    root.setLevel(level)
     root.addHandler(h)
     sh = logging.StreamHandler()
     sh.setFormatter(logging.Formatter("%(levelname)s %(message)s"))