Spirit Airlines filed for Chapter 11 bankruptcy protection for the second time in less than a year last week and announced that it's ending service to a dozen cities in October. Now, rival airlines are swooping in.

On the heels of the bankruptcy news last week, Frontier Airlines, which has a 35% overlap with Spirit on routes, per CNBC , announced it is adding 20 new routes this winter. And on Thursday, United Airlines launched ticket sales for 15 new flights beginning January 6 for Spirit-heavy hubs, such as Chicago, Houston, Las Vegas, Fort Lauderdale, and Orlando.

"If Spirit suddenly goes out of business, it will be incredibly disruptive, so we're adding these flights to give their customers other options if they want or need them," said Patrick Quayle, United's senior vice pre

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