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We present new spectroscopic observations obtained with Gemini Multi-Object Spectrographs at Gemini-South of a sample of 25 H II regions located in NGC 55, a late-type galaxy in the nearby Sculptor group. We derive physical conditions and chemical composition through the T_e method for 18 H II regions, and strong-line abundances for 22 H II regions. We provide abundances of He, O, N, Ne, S and Ar, finding a substantially homogeneous composition in the ionized gas of the disc of NGC 55, with no trace of radial gradients. The oxygen abundances, both derived with T_e and strong-line methods, have similar mean values and similarly small dispersions: 12+log (O/H) = 8.13 ± 0.18 dex with the former and 12+log (O/H) = 8.17 ± 0.13 dex with the latter. The average metallicities and the flat gradients agree with previous studies of smaller samples of H II regions and there is a qualitative agreement with the blue supergiant radial gradient as well. We investigate the origin of such flat gradients comparing NGC 55 with NGC 300, its companion galaxy, which is also twin of NGC 55 in terms of mass and luminosity. We suggest that the differences in the metal distributions in the two galaxies might be related to the differences in their K-band surface density profile. The flatter profile of NGC 55 probably causes in this galaxy higher infall/outflow rates than in similar galaxies. This likely provokes a strong mixing of gas and a re-distribution of metals.