unique field property not maintained

I'm trying to add a unique property to a field but when i do it is not maintained after i access the application and return to the edit the application mode.

When i try to add the unique identifyer in the script

        must have unique   Account_Name
        (
            displayname = "Account Name"
            type = text
            width = 200px
        )

i get the following error:

Error modifying the page Maximum size of a column cannot be reduced, when the table is not empty -- tableName : t_721556000000013544 -- columnName : Account_Na_721556000000013562

I have no records with duplicates in this field, but i have multible records in the form.

I hope you can help me fix this issue.

Thank you tomas