  This paper reports on a new analysis of archival ALMA $870\,\mu$m dust
continuum observations. Along with the previously observed bright inner ring
($r \sim 20-40\,$au), two addition substructures are evident in the new
continuum image: a wide dust gap, $r \sim 40-150\,$au, and a faint outer ring
ranging from $r \sim 150\,$au to $r \sim 250\,$au and whose presence was
formerly postulated in low-angular-resolution ALMA cycle 0 observations but
never before observed. Notably, the dust emission of the outer ring is not
homogeneous, and it shows two prominent azimuthal asymmetries that resemble an
eccentric ring with eccentricity $e = 0.07 $. The characteristic double-ring
dust structure of HD 100546 is likely produced by the interaction of the disk
with multiple giant protoplanets. This paper includes new
smoothed-particle-hydrodynamic simulations with two giant protoplanets, one
inside of the inner dust cavity and one in the dust gap. The simulations
qualitatively reproduce the observations, and the final masses and orbital
distances of the two planets in the simulations are 3.1 $M_{J}$ at 15 au and
8.5 $M_{J}$ at 110 au, respectively. The massive outer protoplanet
substantially perturbs the disk surface density distribution and gas dynamics,
producing multiple spiral arms both inward and outward of its orbit. This can
explain the observed perturbed gas dynamics inward of 100 au as revealed by
ALMA observations of CO.
  Finally, the reduced dust surface density in the $\sim 40-150\,$au dust gap
can nicely clarify the origin of the previously detected H$_2$O gas and ice
emission.
