How to copy data into same table based on existing data in sql?
One more solution is to run sub query for two time with different updated Date_column value. It will select data from same table and updated with different date_value.

INSERT into TABLE_NAME (Column1, Column2, Date_column)( SELECT (Column1, Column2, '201840') FROM TABLE_NAME where Date_column = '201839');

INSERT into TABLE_NAME (Column1, Column2, Date_column)( SELECT (Column1, Column2, '201841') FROM TABLE_NAME where Date_column = '201839');