#377 - remove IDs if there were in schedule_set earlier

Author: Vlad Dobromyslov

libraries/chain/include/graphene/chain/witness_scheduler.hpp

@@ -162,8 +162,12 @@ class generic_witness_scheduler
          _schedule.pop_front();
 
          auto it = _lame_duck.find( result );
-         if( it != _lame_duck.end() )
-            _lame_duck.erase( it );
+         if( it != _lame_duck.end() ) {
+            set< WitnessID > removal_set;
+            removal_set.insert(*it);
+            remove_all( removal_set );
+            _lame_duck.erase(it);
+         }
          if( debug ) check_invariant();
          return result;
       }
@@ -389,7 +393,7 @@ class generic_witness_scheduler
       // scheduled
       std::deque < WitnessID > _schedule;
 
-      // in _schedule, but not to be replaced
+      // in _schedule, but must be removed
       set< WitnessID > _lame_duck;
 };