Pivot Table Duplicate January

I have a column in my original data that uses the formula =TEXT(D11,"mmmm")
to generate the month name, so that I can filter my pivot table by month. For
some reason the filter option on my pivot table gives 3 separate January's
and only one of those actually shows data. The other two show nothing. ( this
only accurs with January, none of the other months are affected) It is no
problem for me, but other people will be using this pivot table to generate
reports and I want to remove the duplicates to eliminate any confusion. Can
anyone help?
Salahan