* (Adds share reaction, analyst comments, acquisition context): MELBOURNE, Aug 23 (Reuters) - Santos Ltd, Australia’s second-largest independent gas producer, reported on Thursday a near doubling in first-half underlying profit and revived its dividend after slashing debt. The turnaround came a day after Santos agreed to buy Quadrant Energy for at least $2.15 billion, expanding in Western Australia and grabbing a potentially big new oil find just three months after rejecting a $10.8 billion takeover offer from private equity-backed Harbour Energy.