Peak separation plotted against the redshift for our double-peak sample and for several studies (Kulas et al. 2012; Yamada et al. 2012; Hashimoto et al. 2015; Matthee et al. 2018; Songaila et al. 2018; Bosman et al. 2020; Meyer et al. 2021; Naidu et al. 2022). Our double-peak sample represented in purple shows three different symbols for the three categories GOLD (triangle), SILVER (cross), and BRONZE (square). A horizontal black dash-dotted line at 150 km s⁻¹ represents the peak separation threshold made in Sect. 4.1.2 to obtain the fraction of double-peaks. The peak separations are in discrete lines due to the fact that the minimal peak separation measured is two spectral bins and depends on redshift.