Duplicated legend labels are not allowed

Legend labels are merged by name. Example:

```
data = pd.DataFrame({
'category': ['A', 'B', 'C', 'D'],
'value': [10, 15, 20, 25]
})

ggplot(data) + \
geom_point(aes(x='category', y='value', color='category')) + \
scale_color_manual(values=['red', 'blue', 'green', 'black'], labels=['Label A', 'Label B', 'Label A', 'Label D'])
```

Output:

Expected: