If we consider that 1/6, in place of 1/12, mass of carbon atom is taken to be the relative atomic mass unit, the mass of one mole of the substance will  

Relative atomic mass =
$\frac{\mathrm{Mass}\mathrm{of}\mathrm{one}\mathrm{atom}\mathrm{of}\mathrm{the}\mathrm{element}}{1/12 \mathrm{part}\mathrm{of}\mathrm{the}\mathrm{mass}\mathrm{of}\mathrm{one}\mathrm{atom}\mathrm{of}\mathrm{Carbon}—12}$
$\text{ or }\frac{\text{ Mass of one atom of the element }}{\text{ mass of one atom of the }\mathrm{C}-12}\times 12$
Now if we use 1/6 in place of 1/12 the formula becomes 

Relative atomic mass  =
$\frac{\text{ Mass of one atom of element }}{\text{ Mass of one atom of carbon }}\times 6$
$\therefore$Relative atomic mass decrease twice