How to dry out rehydrated dried mangos?
Doug has the right idea, but ovens trap the moisture, increasing the humidity around the food, decreasing the efficiency of the oven as a drying machine. 

For detailed information regarding the physics of drying foodstuffs, I found the following helpful: http://www.nzifst.org.nz/unitoperations/drying1.htm

From these ideas, I would suggest placing the mangoes in a dry ventilated room, with some sort of heat source applied (doesn't have to be much). The combination of the dry air and the heat will speed the evaporation.