Can I put a new flat roof on top of the old flat roof?

Usually no. There are many factors that determine if you can put a new roof over an existing flat roof.  One factor is if it is required to remove the old roof by local, state or national building codes. Other considerations are what the condition of the insulation and decking are and also if you have existing water saturation. If there is reason to believe there is water damage to insulation and decking, the old flat roof should be removed to make proper repairs.