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Ryan Phillippe might have to prep for round three of Daddy Duty!

The actor, 36, who already has two kids with ex-wife Reese Witherspoon, may become a father again -- so says his ex, pregnant model-actress Alexis Knapp, who has claimed she's carrying his baby. (Knapp recently stepped out in Hollywood, bearing an unmistakable baby bump.)

"She's in the second trimester," a source close to Knapp confirms to of Knapp, who next stars in the  film Project X (out this Thanksgiving) plus the Miley Cyrus flick So Undercover. (A rep could not be reached for comment.)

First-time mom Knapp dated Phillippe for several months on and off last year before he ended the relationship in September. "She has been telling people he is the father," explains another source to Us.  Counters the Knapp source: "She's not commenting on who the father is."

"Ryan doesn't know if he is the dad," adds the second source of the Lincoln Lawyer actor. "If he is, he will take full responsibility for the child."

Explains a third insider of Phillippe and Knapp: "He and Alexis were actually together for a while...It was pretty serious."

"He's a good guy," adds the insider. "He loves being a dad and takes being a dad really seriously."

His kids with Witherspoon, 34, are Ava, 11, and Deacon, 7.

"If this kid is his," the insider says, "He will do the right thing. He takes his responsibility seriously."

Phillippe's current squeeze is Amanda Seyfried -- although the duo have been tight-lipped about their romance. "I'm dating," said Phillippe on Friday's Ellen DeGeneres Show. "Not nearly as voracious as the magazines like to make out, [but] I am dating."