MySQL unique index has no duplicate value, and the error is repeated

Here is the error report max_index This unique index duplicate value

Duplicate entry '12345678999' for key 'max_index'

  In fact, now the database No, '12345678999'  This value

as a result of my where The latter condition can find more than two pieces of data   , Change these two or more pieces of data to m=12345678999  And this m It's the only index So wrong reporting This unique index repeat