Source: Hart, p. 180; Whitney §1000ff.
This secondary form is not common in classical Sanskrit except for a few roots. Like the desiderative, it can take all forms, but is rarely seen outside the present. It means to repeat or intensify the action of the verb. In the present it is inflected like verbs of class III (3), and its stem is formed from a strong and unusual reduplication.
Thus from विद्, the intensative 3rd singular present active is वेवेत्ति or वेविदीति, “he really knows” or “he knows indeed.”