Extracting Copper from Copper Carbonate
The repeated heatings were just to ensure that you had oxidized all of the copper carbonate to copper oxide. If you heated it a second time and the mass decreased, that would just mean that some of the copper carbonated had not been oxidized by the first heating. Then you would just keep heating until the mass quit decreasing, meaning that you had definitely oxidized all of it.  

So the answer to you question is simply that the mass theoretically should not have decreased at all with a second heating, unless some of the copper carbonate had not been oxidized the first time.