Why does the representation of sparse matrices allow for duplicates?

```
a = spzeros(2, 3)
a[[1,1],2]=1
a[[1],2]=2
```

Leads to

```
2×3 sparse matrix with 2 Float64 nonzero entries:
[1, 2] = 2.0
[1, 2] = 1.0
```

This behavior seems prone to computation errors. Why is this allowed? What is the use case?