We add a custom field, pointing to an existing attribute, to a form such as detail_in.htmpl. When we open the form, the message 'Attribute xxx with caption "xxx" set readonly - already on page with caption "xxx"' appears as seen in the following screenshot.
The message is displayed if the same attribute is pointed to by both, a custom field and an existing field on the form. The message is displayed in order to keep data integrity.
On the Incident Detail screen above, the custom form field with a Caption "Outage Start Time", which points to the attribute "outage_start_time" has been added. The attribute "outage_start_time" has already been referenced by by the out-of-the-box form field, "Start Time" on the [Additional Information] > [Outage] tab. In this case, the out-of-the-box field "Start Time" is set to Read-Only.
In order to avoid the out-of-the-box field becoming Read-Only or to store different data in each field, create a new custom column and specify it as the attribute of a custom field or remove the existing default field from the form.