MySQL 9.7

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create table franck ( val int unique );
insert into franck values (-1),(1) ;

Records: 2  Duplicates: 0  Warnings: 0

select val from franck;

val

-1

1

update franck set val=-val;

Duplicate entry '1' for key 'franck.val'

select val from franck;

val

-1

1