--- translate/storage/base.py	2011-05-15 20:14:19.000000000 +0200
+++ /home/claude/.../translate/storage/base.py	2012-03-13 14:17:21.759808031 +0100
@@ -543,6 +543,9 @@
         @param unit: The unit that will be added.
         """
         unit._store = self
+        # Prevent two header units in the same store
+        if unit.isheader() and len(self.units) and self.units[0].isheader():
+            self.units[0] = unit
+        else:
+            self.units.append(unit)
-        self.units.append(unit)
 
     def addsourceunit(self, source):