Reports that have duplicate column names have this because the column list in the underlying query are using the same names and / or alias.
- E.g. Both of the following columns are using the alias "NAME".
select USER.USER_NAME as NAME, USER.FULL_NAME as NAME from USER
- E.g. More realisticaly, in this example you might erroneously assume that you can pick any alias, but you have actually duplicated the alias USER_NAME since * would have a reference to all columns in the table, one of which is USER_NAME.
select *, USER_NAME from USER
Simply make sure that your names are unique
Note: If you are using the Auto-generated layout (most of you are) then the column names shown on the reports comes from the Aliases of the columns as well.