code: private void checkForDuplicateConditions(List<Stmt.Case> cases) {
  		HashSet<Expr> seen = new HashSet<>();
  		for(int i=0;i!=cases.size();++i) {
  			Stmt.Case c = cases.get(i);
  			Tuple<Expr> conditions = c.getConditions();
  			// Check whether any of these conditions already seen.
  			for(int j=0;j!=conditions.size();++j) {
  				Expr condition = conditions.get(j);
  				if(seen.contains(condition)) {
  					syntaxError("duplicate case label", condition);
  				} else {
  					seen.add(condition);
  				}
  			}
  		}
  	}
docs: Check whether we have duplicate case conditions or not. Observe that this is
  relatively simplistic and does not perform any complex simplifications.
  Therefore, some duplicates are missed because they require simplification.
  For example, the condition <code>1+1</code> and <code>2</code> are not
  considered duplicates here.  See #648 for more.