A period similar to the parallel period, but in which there are two antecedent and two consequent phrases. The result can be diagrammed as
Here, an antecedent phrase (a) ending in a relatively inconclusive cadence (an IAC or HC) is followed by a consequent (b) whose cadence is weaker than expected. The antecedent is thus repeated, then followed by a modification of the consequent, which this time ends with a conclusive cadence (and is thus represented as b', since it is similar to the first consequent, but ends in a different sort of cadence).
Mozart loves to use double period form for the secondary themes of his sonata-form works.