In a multicolumn cell which doesn't include the first column of the tabular,
you should not have an option to enable left border (only right border).
Here is an explanation why (from array.sty documentation):
As noted by the \LaTeX{} book, for the purpose of
"\multicolumn" each column with the exception of the first one
consists of the entry and the \emph{following} inter-column
material. This means that in a tabular with the preamble
"|l|l|l|l|" input such as "\multicolumn{2}{|c|}" in
anything other than the first column is incorrect.
In the standard array/tabular implementation this error is not so
noticeable as that version contains negative spacing so that each
"|" takes up no horizontal space. But since in this package the
vertical lines take up their natural width one sees two lines if
two are specified.
