Remove existing logging handlers before adding a new one

This fixes a bug where log messages are printed multiple times when
xfaster_run() is called multiple times in the same python session.

xfaster/xfaster_class.py

@@ -438,6 +438,10 @@ def root_notice(msg, *args, **kwargs):
 
         # configure logger
         self.logger = logging.getLogger("xfaster")
+        # replace any existing handlers before adding one
+        if logger.hasHandlers():
+            for h in list(logger.handlers):
+                logger.removeHandler(h)
         self.logger.addHandler(handler)
 
         # set logging level