Top panel: Histogram distribution of the difference between the systemic redshift estimated from C III] performing a single-Gaussian fit (z_{sys, single}) or a double-component fit (z_{sys, double}). The blue histogram is the sum of all galaxies with C III] detected at S/N ≥3. The yellow and green histograms represent the subsets where the best-fit flux ratio between the two components of the double-Gaussian fit reaches one of the two extremes (respectively 1 and 0.625). The red histogram, from which we derive the correction of 26.25 km/s for the systemic redshift estimated through a single-Gaussian fit, includes galaxies with a best-fit ratio within the allowed range. Bottom panel: Velocity difference between the systemic redshift estimated from a single-Gaussian and a double-Gaussian fit of the C IIIλλ1907-1909 doublet, from Monte Carlo simulations, as a function of the S/N of the C III] line. The horizontal green dashed line represents the median difference for the simulated dataset.