diff --git a/ess/TraceWin.py b/ess/TraceWin.py
index 28c66bdcf2f4ac057f6ec211837c809bc50cfebe..4eaf6dddb47ed03b1c8b6a704ae3cc7fa80eeb51 100644
--- a/ess/TraceWin.py
+++ b/ess/TraceWin.py
@@ -718,7 +718,9 @@ class density_file:
         '''
         import h5py
 
-        fout = h5py.File(filename,'w')
+        fout = h5py.File(filename,'a')
+        if group in fout:
+            del fout[group]
         group = fout.create_group(group)
 
         # header attributes..