Trouble with selecting a single column.

I am trying to add a column onto the end of a table and having much
difficulty, when I select the table and add "columns to the right it adds"
more than one column and when I try to delete the additional column for some
rows (but not all) it deletes both columns. Am having the same problem when
I try to select just the last column (using the down arrow at the top of the
column) on some rows more than one column will be selected. I don't see any
special formatting when I use the "Show/Hide" button. Any ideas?