Graph Categories Repeating

There are many reasons why experienced users of spreadsheets recommend against using merged cells. What you have is the same text in column a and b, and you are using both column a and column b ( because they are merged). If you unmerge the cells and delete either column a or column b, then you will only get one category label. Or, you can unmerge the cells and select either column a or column b to be the vertical axis labels rather than both columns. Or something similar to either.