When the `search` plugin is enabled, with the options:

```
# search plugin
# path to the omega cgi program
omega_cgi: /usr/lib/cgi-bin/omega/omega
# use google site search rather than internal xapian index?
google_search: 0
```

Then we see the following error when editing a page from the web browser UI or running `ikiwiki --rebuild`:

```
Error: Exception: Db block overwritten - are there multiple writers?
```

The issue goes away when we disable the 'search' plugin.

We're running ikiwiki version 3.20180228-1.