_col=id can cause id column to export twice in CSV export

https://datasette.simonwillison.net/simonwillisonblog/blog_entry.csv?_col=id&_col=title&_col=body&_labels=on&_size=1

```csv
id,id,title,body
1,1,WaSP Phase II,"<p>The <a href=""http://www.webstandards.org/"">Web Standards</a> project has launched Phase II.</p>"
```
That should not have two `id` columns.