copying data down to empty cells below until next new value

I have a worksheet with thousands of rows of data. In column a it is Part # and Column B is Quantity. There are multiple rows for each Part #, but the Part # is only input on the first line and none of the corresponding lines. I am needing to copy down the Part # to all of the blank cells below it and repeat for each new Part #. I am having to do this, because in the end I will be doing an Advanced Row Combine based on Part # and have it add the Quantity. Is there a much easier way to do this rather than me constantly copying and pasting or dragging down to copy the cells?

Thanks,
Chad