Potential killer said to be stupid, repeatedly (6)

I believe the answer is:

dumdum

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'potential killer' is the definition.
(dumdum is a type of bullet)

I cannot understand how the rest of the clue works.

(Other definitions for dumdum that I've seen before include "Soft-nosed bullet type" , "Soft-nosed bullet that expands when it hits target" , "missile" , "Exploding type of bullet" , "Illegal soft-nosed bullet" .)

I've seen this clue in The Times.

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