The doubling effect is a studio recording technique often used to ``thicken'' vocals in which the same part is sung twice by the same person. In other words, doubling is a ``chorus of two'', where both parts are sung ``in unison'' by the same person. As an example, the Beatles used doubling very often, such as on the track ``Hard Day's Night''. A single variable delay line can simulate doubling very effectively.
The related term slap back refers to the use of a single echo on a recorded track. The echo is often placed in a different spatial location in the stereo mix. Normally the echo delay is just large enough to be heard as a discrete echo on careful listening (e.g., on the order of tens of milliseconds). Compared to doubling, slap back uses a larger delay relative to the first voice, and the delay need not vary.