RATIONALE: Giving chemotherapy drugs, such as melphalan, work in different ways to stop the growth of cancer cells, either by killing the cells or by stopping them from dividing. Chemoprotective drugs, such as amifostine, may protect normal cells from the side effects of chemotherapy. Giving chemotherapy with a peripheral stem cell transplant once or twice, using stem cells from the patient or an identical brother or sister, may allow more chemotherapy to be given so more cancer cells are killed. Giving maintenance therapy after a stem cell transplant may kill any cancer cells that remain. It is not yet known which dose of melphalan is more effective in treating multiple myeloma (MM).