We’re using a single table to record daily data – the date field is set as the
pk so only one record per date is allowed.
If the user enters a date (when creating a new record) that
already exists in the table what is the best way to bypass the
Access error message about duplicate values and allow the
user to enter a new date?
(So instead the of getting the error message the user is simply
prompted to re-enter the date)

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Put logic in the form's BeforeUpdate event to check whether the date already exists in the table (use DLookup, for example)