CREATE TABLE table_204_790 (id DECIMAL, "year" DECIMAL, "nw rank" DECIMAL, "venue" TEXT, "opp rank" DECIMAL, "opponent" TEXT, "score" TEXT, "w/l" TEXT, "round" TEXT, "notes" TEXT)

who was an opponent in 1996 and 1997 ?
SELECT "opponent" FROM table_204_790 WHERE "year" = 1996 INTERSECT SELECT "opponent" FROM table_204_790 WHERE "year" = 1997
SELECT "opponent" FROM "table_204_790" WHERE "year" = 1996 INTERSECT SELECT "opponent" FROM "table_204_790" WHERE "year" = 1997