MIDEAST STOCKS-Egypt tumbles on stamp duty worry, Q4 earnings support Saudi

* Sources say Egypt may introduce stamp duty * Saudi's top two banks gain on strong Q4 * SABIC slips as Q4 net profit at lower end of forecasts * Zain jumps on narrower loss, Mobily down on loss * Mouwasat Medical falls despite earnings beat * Kuwait strong as liquidity builds By Celine Aswad DUBAI, Jan 19 (Reuters) - Egypt's blue-chip stock index tumbled on Thursday after Reuters reported that authorities were considering temporarily reintroducing a stamp duty on stock market transactions, whil