Stop making notes about once-forgotten features.

The game makes a note of some features whenever they are marked as
"seen". The "forget level map" command clears the "seen" flag for most
of the features on the current level, so something can have several
notes made about its discovery.

Only make a note the first time something is seen.

crawl-ref/source/map-knowledge.cc

@@ -126,7 +126,9 @@ void set_terrain_seen(const coord_def pos)
     {
         _automap_from(pos.x, pos.y, _map_quality());
 
-        if (!is_boring_terrain(feat))
+        if (!is_boring_terrain(feat)
+            && (!env.map_forgotten
+                || ~(*env.map_forgotten)(pos).flags & MAP_SEEN_FLAG))
         {
             string desc = feature_description_at(pos, false, DESC_A) + ".";
             take_note(Note(NOTE_SEEN_FEAT, 0, 0, desc));