[moz]

First, the months are displayed twice because of this:

The combobox is created with a string[] parameter of values.

inputMonth = new JComboBox(months);

Then more values are put in the list (duplicating the entries).

for (int i = 0; i < 12; i++)
{
     inputMonth.addItem(months[i]);
}

JComboBox has one constructor like this:

JComboBox(Object[] items) 
Creates a JComboBox that contains the elements in the specified array

So you can either do this:

inputMonth = new JComboBox(); // No parameter
for (int i = 0; i < 12; i++) // Fill in values
{
     inputMonth.addItem(months[i]);
}

or simply this, as you have done and lose the for loop:

inputMonth = new JComboBox(months);

I'll look at the rest if I get time