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Erik Menendez is on the mend after going under the knife for kidney stones ... TMZ has learned.

A rep for the California Department of Corrections and Rehabilitation tells TMZ ... Erik was transported to an outside medical facility on September 26. They tell us he's since been discharged and is back at Richard J. Donovan Correctional Facility in San Diego, where he remains in good condition.

A source confirmed to us that Erik was transported to an outside medical facility to undergo treatment for kidney stones.

Erik's wife, Tammi , also hinted at the health scare earlier this week. She posted a video on Tuesday, and when someone asked in the comments how Erik was doing, she replied, "He just had another surgery but recovering. He is doing okay."

We broke the story ... Erik wa

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