UNION or DISTINCT()?

When deciding whether to use DISTINCT() or UNION in a query to remove duplicate rows, which of the following questions would you NOT ask yourself?

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Possible Answers

Is there an alternative method to using DISTINCT()?
press1
Will appending queries produce duplicate rows?
press2
Is there an aggregate function in the SELECT statement?
press3
Should I be thinking about duplicate rows because my queries never produce duplicate rows?
press4