:: This is the same behaviour as when you change a domain so it
:: has a default value. The column may contain nulls. Good
:: admin tools realise this and ask if you want to update the
:: field. Likewise it's up to you before creating a unique
:: index to check for duplicates. It's not just the presence of

I understand this, but my field did not have any duplicates. The only
duplicate was "null" which is not a value and therfore should not be
considered.

:: null values it's duplicates of any kind.

I didn't have any duplicates.

:: I've never considered this a bug - just an administrative
:: task to carry out when you are (re)designing your database.
:: select mynewunique, count(*) from mytable group by
:: mynewunique will show you straight away if what you are
:: proposing is sensible given the current state of that table.

In fact to be sure, I just ran this now and there we no duplicates, only
null. Null has 1213 occurrances, everything else had 1.