Evidence of the unique source and double lens nature of J1721+8842. Top: NOT light curves of the D and E lensed images of J1721+8842. The inset is a colour-composite HST/WFC3 image of the system with the lensed components labelled. The two light curves are identical within their noise envelope, with D preceding E by about 35 days. Bottom: JWST/NIRSpec spectrum of the far deflector arced by the foreground lens. The spectrum was extracted from the mask drawn on the inset, which itself is a single slice of the NIRSpec cube. The redshift was determined from the overlaid absorption lines. Note that the total flux presented here is the one observed, not corrected for the magnification due to lensing.