Doppio movimento means double the speed — the tempo doubles from whatever came before. It is a precise instruction rather than a gradual change, requiring an immediate shift to twice the previous pace.
This marking appears most often at structural transitions where a composer wants a dramatic change of energy. Verdi used it frequently in his operas to shift between reflective passages and moments of action. Unlike accelerando, which is gradual, doppio movimento is instantaneous — the ensemble must move to the new tempo as one.
Verdi used doppio movimento in Rigoletto to create one of opera's most dramatic gear changes — the music suddenly doubles in speed as the plot takes a fatal turn.