## Explanation

This formula uses COUNTIF with an expanding range to first check if the current row is the first occurrence of a given invoice number:

```
COUNTIF($B$5:B5,B5)=1
```

This expression only returns TRUE when this is the first occurrence of a given invoice number. If so, a SUMIF calculation is run:

```
SUMIF($B:$B,B5,$D:$D)
```

Here, SUMIF is used used to generate a total sum by invoice number, using the amounts in column D.

If the count is not 1, the formula simply returns an empty string ("")