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For the second time in less than two weeks, major U.S. airlines are raising fares.

United Airlines confirmed Thursday that it increased prices on many U.S. flights by $10 per round trip.

J.P. Morgan airline analyst Jamie Baker says, however, that he’ll be surprised if Southwest follows suit, coming only two weeks after airlines raised base fares by $4 to $10 per round-trip ticket.

If discount airlines such as Southwest refuse to go along, United could be forced to retreat.

Also, airlines often undercut price increases by running fare sales. For example, Delta boasted about weekly specials in an e-mail to customers on Tuesday.