Systems of quantum dots connected to leads exhibit periodic conductance peaks as a function of gate voltage arising from the Coulomb blockade effect. Very surprisingly, we find that in the intermediate coupling regime the conductance through a metallic nanoparticle is characterized by two gate voltage periods. The relative strength of the two periods depends both on the dot-lead coupling the source-drain voltage and the dot size. Our results exclude the presence of two distinct dots or interference effects due to several electronic trajectories as a physical origin of two periods. On the other hand the measurements reinforce the scenario that the second period is due to electronic surface states and we might have inherently two-dots (surface states and bulk states) within a single metallic dot.