/.)a+b+c /. c->d
g[a+b+c,a]/.g[x_+y_,x_]->{x,y}
If rules is a list of lists, a list of all possible respective replacements is returned:
{a, b} /. {{a->x, b->y}, {a->u, b->v}}
The list can be arbitrarily nested:
{a, b} /. {{{a->x, b->y}, {a->w, b->z}}, {a->u, b->v}}
{a, b} /. {{{a->x, b->y}, a->w, b->z}, {a->u, b->v}}