Plasma non-uniformities in the outer part of the Sun's atmosphere (the corona), such as coronal loops, are known to support a variety of magnetohydrodynamic (MHD) waves. A commonly detected oscillation mode is transverse displacements of the loop, called kink oscillations. Low-amplitude kink oscillations are known to persist for many periods, despite theoretically being subjected to damping. These ubiquitous, so-called "decay-less" oscillations remain enigmatic. In this work, we used the innovative movie processing technique of "motion magnification" to study decay-less kink oscillation in a well contrasted loop. Two periods were found to be present: P_1 = 10.3 (+1.5,-1.7) minutes and P_2 = 7.4 (+1.1,-1.3) minutes. Moreover, the shorter period oscillations are absent at the loop top, and are in anti-phase in its legs. This behaviour is interpreted as the coexistence of the fundamental and second harmonics of the decay-less kink mode of the loop. The existence of multiple harmonics may help us understand the driving mechanism of these decay-less oscillations. More generally, the observational detection of multiple kink harmonics opens up excellent perspectives for the routine diagnostics of the coronal plasma, necessary for the robust forecasting of space weather extreme events.