It's possible that the original will may specify a 'survival period' which has to apply before the intended beneficiary receives a share of the estate. If so, and that person dies before the end of this period, then the original will should state what is to happen to that part of the estate.
If the above circumstances do not apply, then the bequest will form part of the newly-deceased person's estate (even though s/he's never actually received it). It will therefore form part of the new estate which is to be distributed under the terms of the newly-deceased person's will (or, if no will exists, in accordance with the intestacy rules).
Chris