It could be possible that you renamed column names to the same name using the AS clause. Here is the explanation from IBM information center
A column list must be specified in a CREATE VIEW statement, a common table expression or in a CREATE TABLE statement containing an as-subquery-clause when:
- any element of the SELECT list in the fullselect is other than a column name and is not named using the AS clause.
- any two elements are the same column name that were not renamed using the AS clause.