** Shares of Frontier Group Holdings ULCC.O up 18% to $5.8 in early trade after Deutsche Bank upgrades rating on the stock to "buy" from "hold"

** Says it sees the low fare carrier as best-positioned to be the biggest beneficiary of Spirit Airlines' FLYY.A bankruptcy given their network overlap

** Last week, Spirit filed for bankruptcy protection for the second time in a year

** Brokerage says it estimates that the overlap should increase to about 40% by the December quarter following Frontier's recent announcement to add 20 new routes beginning later this year

** Median PT of 11 brokerages covering the stock is $5.50 - LSEG data

** As of last close, stock down 31% YTD

(Reporting by Anshuman Tripathy)

((Anshuman.Tripathy@thomsonreuters.com))