Merge CSV files into a single file with no repeated headers

I have some CSV files with the same column headers. For example

File A

header1,header2,header3
one,two,three
four,five,six

File B

header1,header2,header3
seven,eight,nine
ten,eleven,twelve

I want to merge it so that the data is merged into one file with the headers at the top, but no headers anywhere else.

header1,header2,header3
one,two,three
four,five,six
seven,eight,nine
ten,eleven,twelve

What is a good way to achieve this?