Most bulbs multiply both by shedding seed and producing offsets from the bulb itself. Case in point this Ramson (Allium ursinum) was one bulb when I planted it and has now become two. 

 Ramsons spread very vigorously when the conditions are right; damp deciduous shade is their natural habitat as seen here in a Wiltshire wood. The air reeks of garlic when they carpet the ground like this!