The sirens sound more than once.
From the text:
The first warning siren went off in the morning while we were at breakfast, but we
paid no attention, because it only meant that the planes were crossing the coast.
At two-thirty Margot had finished her office work and was just gathering her things
together when the sirens began wailing again. So she and I trooped back upstairs.
None too soon, it seems, for less than five minutes later the guns were booming so
loudly that we went and stood in the hall. The house shook and the bombs kept falling.
After the dishes had been washed: another air-raid warning, gunfire and swarms of
planes. "Oh, gosh, twice in one day," we thought, "that's twice in one day," we
thought, "that's twice too many." Little good that did us, because once agai the bombs
rained down, this time on the others of the city.