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Kroy Biermann reached out to the woman who subpoenaed Kim Zolciak in her divorce case to warn her about Kim ... this according to the woman's deposition in that case.

Jillian Green was deposed in the case back in June ... and she testified Kroy actually called her to share his concerns about Kim's relationship with Jillian's estranged husband, Kyle Mowitz . Gallery Launch Gallery

Green claimed Biermann told her Zolciak would "drain money from Kyle" ... saying Kim already put deliveries of groceries from Instacart on Mowitz's credit card.

Jillian said Kroy warned her about Kim's mental state ... saying he called her a narcissist with a gambling problem who was never around for her children -- assertions which all mirror past legal documents he filed during their divo

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