Here is solution if Unique Weight was number 10 and Heavy

(followed by solution if it was Light).

Weigh #1

Place 1 2 3 4 on Left Place 5 6 7 8 on Right

Scale would be in balance so Solver knows Unique is either 9 10 11 or 12

Weigh #2

Place 1 and 9 on Left and Place 11 and 12 on Right

Scale would be in balance

So Unique weight must be 10 as Heavy or Light

weigh#3

Solver would place 9 (a known Standard) on one side and 10 on the other side.

It would reveal that 10 is Heavy.

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Now assume instead that 10 is Light

All 3 Weighs would be as above except now Weigh#3

would reveal that the unique weight number 10 is Light.

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