Equivant mass of double salt
$\ce{CaCO3.MgCO3}$ reacts with $\ce{4 HCl}$, therefore $m_\mathrm{eq}=m/4$.
$\ce{CaCO3}$ reacts with $\ce{2 HCl}$, therefore $m_\mathrm{eq}=m/2$.
$\ce{MgCO3}$ reacts with $\ce{2 HCl}$, therefore $m_\mathrm{eq}=m/2$.
$\ce{Na2CO3.10 H2O}$ reacts with $\ce{2 HCl}$, therefore $m_\mathrm{eq}=m/4$.
$\ce{1 CO2}$ of carbonate origin is released by $\ce{2 HCl}$, therefore $m_\mathrm{eq}=m/2$.