I thought UNION didn't select duplicate records?

I'm using this huge SQL statement:

strSQL = "(SELECT Tasks.*,TasksCompleted.*" _
& " FROM TaskListItems" _
& " INNER JOIN (Tasks " _
& " LEFT JOIN TasksCompleted ON Tasks.ID = TasksCompleted.TaskID)" _
& " ON TaskListItems.TaskID = Tasks.ID" _
& " WHERE (TaskListItems.TaskListID=" & TaskListID _
& " AND TasksCompleted.UserID=" & CurUser.ID _
& " AND TaskListItems.Removed=False" _
& " AND Tasks.AFSCID=" & CurUser.AFSC.ID _
& " AND Tasks.ParentID=" & ID & "))" _
& " UNION" _
& " (SELECT Tasks_1.*,TasksCompleted_1.* FROM Tasks AS Tasks_1 LEFT JOIN TasksCompleted AS TasksCompleted_1 ON Tasks_1.ID = TasksCompleted_1.TaskID" _
& " WHERE Tasks_1.Heading=True" _
& " AND Tasks_1.AFSCID=" & CurUser.AFSC.ID _
& " AND Tasks_1.ParentID=" & ID & ")" _
& " UNION" _
& "(SELECT Tasks_2.*,TasksCompleted_2.*" _
& " FROM TaskListItems AS TaskListItems_2" _
& " INNER JOIN (Tasks AS Tasks_2 " _
& " LEFT JOIN TasksCompleted AS TasksCompleted_2 ON Tasks_2.ID = TasksCompleted_2.TaskID)" _
& " ON TaskListItems_2.TaskID = Tasks_2.ID" _
& " WHERE (TaskListItems_2.TaskListID=" & TaskListID _
& " AND (TasksCompleted_2.UserID<>" & CurUser.ID _
& " OR TasksCompleted_2.UserID Is Null)" _
& " AND TaskListItems_2.Removed=False" _
& " AND Tasks_2.AFSCID=" & CurUser.AFSC.ID _
& " AND Tasks_2.ParentID=" & ID & "))"

But in some cases, it returns duplicate records from the "Tasks" table. I thought UNION only selects distinct records, while UNION ALL would create duplicates?

a UNION query does not eliminate "duplicates" from one of the queries involved in a join

a UNION query eliminates duplicate result set rows

this depends entirely on the values in the columns of the SELECT list, nothing else

by the time the UNION sort takes place -- you realize it is sorting your entire result set, right? -- it no longer matters which table a particular result set column came from

do yourself a favour, run your queries separately, and save the output of each one separately

if your final result set appears to have duplicate rows, take another look, there is bound to be something different in one of the columns