The ClarisWorks database environment only allows you to insert a field onto a layout once. You cannot have multiple copies of a field on the same layout.
If when Inserting a Field onto a layout, the field does not show up in the list, it is already on the layout. If you cannot find the field on the layout, it may be outside the page boundaries. When you change page orientation, fields may fall out of the page boundary. It is also possible to move fields off the edge of the layout, especially by using "Select All" and dragging something down from the top of the layout.
To find fields which may be outside the page boundary, try switching page orientations or page sizes until they become visible. Another solution is to just create a new layout.