JUNO is a Liquid Scintillator Antineutrino Detector (LAND) currently under construction in the south of China (Jiangmen city, Guangdong province). Once completed, it will be the largest LAND ever built, consisting in a 20 kt target mass made of Linear Alkyl-Benzene liquid scintillator, monitored by roughly ~18000 twenty-inch high-QE photomultipliers (PMTs) providing a ~80% photo-coverage. Large photo-coverage and large QE are two key requirements of the experiment to yield ~1200PE/MeV needed to achieve the potentially unprecedented ~3% total energy resolution at 1 MeV. This is mandatory to determine the neutrino mass ordering. In order to address the systematic uncertainty challenge causing the non-stochastic component of the energy resolution, the JUNO collaboration conceived a novel neutrino detector design comprising a second layer of small PMTs. This is geared to provide a second calorimetry handle with complementary systematic budget, allowing a combined, more precise and more accurate energy scale definition. We refer to this calorimetry redundancy system as double-calorimetry. In my talk, I will review the motivations which led us to introduce double-calorimetry in JUNO, including the trailblazing capabilities of this approach, as well as the technical challenges associated to its realisation and implementation.