I'm new to product photography, what causes this blown out highlight in the mannequin's left hand as shown in the picture above?
See the lighting setup below, I am using two softboxes as light modifiers.
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Hum... I have the feeling you are sacrificing your overall ilumination for some natural reflection high lights. Your image looks too dark.
A reflective material like glossy plastic is expected to have thoose blowned highlights.
On this post Glossy "metallic" looking skin someone needed the opposite thing, a normal skin image to look glossy. Look at the graph. The intention is to blow thoose reflections.
If you need a matt mannequin, you need either re-paint it, or blur that in Photoshop. Probably some HDRI technique. But in my opinion you do not need to do that.
As can be seen, this is a direct reflection of the flash, it basically renders the image of the softbox distorted by the surface of the mannequin on the hand.
I am not a product photographer, though, these are just some ideas that came to mind.