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Sia 's divorce from her second husband is getting nasty ... because a custody battle just broke out with both of them hurling accusations at the other.

In new legal docs, obtained by TMZ, Sia's estranged husband Daniel Bernard is asking the judge for sole custody of their one-year-old, Somersault Wonder Bernard , alleging she is a "serious and immediate danger" to their son, but Sia says he's already restricted to monitored visits because of a child porn investigation. Shutterstock Premier

In the papers, Daniel says, "Sia is an unfit and unreliable parent struggling with substance abuse and addiction, rendering her incapable of providing safe or stable care for Summi. I am the only safe and reliable parent for our son. I am a doctor, young, healthy,

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