This bug only occurs on 4-2-stable. The conditions to make it occur
require doing a number of pretty dirty things. All of the following must
be true (roughly in order)

- The column is of a type which coerces to a complex data structure
- The data structure has mutability which is not protected by a shallow
  copy
- You are calling `.dup` on records which have never read or assigned
  that field
- You are mutating values deep inside the datastructure on records which
  have never had a value assigned

The reason this manifested through `.dup` on an Active Record model was
due to how we calculated changes in 4.2, which forced values to be
assigned on the defaults, and as per our documentation we only perform a
shallow copy of attributes, not a deep one. This issue does not occur on
5.0 and later due to sweeping changes in how duplication and dirty
tracking occur.

Fixes #25954