Young supernova remnants provide rare glimpses of material from the cores of massive stars, leading to observational tests of stellar evolution and nucleosynthesis theories. Here we present narrow-band images of an unusual swirl structure located near the centre of the Puppis A supernova remnant, which reveal three overlapping filamentary systems of highly diverse composition. One is dominated by emission lines of nitrogen, one by oxygen and one by sulphur. Spectra indicate that these small rings have high velocities and a corresponding kinematic age of less than 800 yr. Heavy-element abundances are extremely high and different in each system, some reminiscent of knots in Cas A, and others of circum-stellar matter observed surrounding supernova 1987A. The youth and unusual chemistry lead us to suggest that a second supernova has exploded within the shell of Puppis A, giving rise to the swirl structure. A similar suggestion was made by Elliot et al.1, based only on the region's appearance in Hα.