Phantom posts appearing when editing config.yaml

I’ve been using hugo serve to live preview my site as I migrate to Hugo, but I’ve found that every time I make a change to my config.yaml I get more and more copies of each post/page appearing. Specifically, the first edit causes one new copy, and each edit after that seems to cause two more. This shows on the index page, which lists all of the copies (with the same link), and in the console log which looks a bit like this:

ERROR: 2016/08/11 09:09:48 site.go:1234: Two or more menu items have the same name/identifier in Menu "main": "About me".
Rename or set an unique identifier.
ERROR: 2016/08/11 09:09:48 site.go:1234: Two or more menu items have the same name/identifier in Menu "main": "About me".
Rename or set an unique identifier.
ERROR: 2016/08/11 09:09:48 site.go:1234: Two or more menu items have the same name/identifier in Menu "main": "About me".
Rename or set an unique identifier.
ERROR: 2016/08/11 09:09:48 site.go:1234: Two or more menu items have the same name/identifier in Menu "main": "About me".
Rename or set an unique identifier.
ERROR: 2016/08/11 09:09:48 site.go:1234: Two or more menu items have the same name/identifier in Menu "main": "About me".
Rename or set an unique identifier.
ERROR: 2016/08/11 09:09:48 site.go:1234: Two or more menu items have the same name/identifier in Menu "main": "About me".
Rename or set an unique identifier.
ERROR: 2016/08/11 09:09:48 site.go:1234: Two or more menu items have the same name/identifier in Menu "main": "About me".
Rename or set an unique identifier.
ERROR: 2016/08/11 09:09:48 site.go:1234: Two or more menu items have the same name/identifier in Menu "main": "About me".
Rename or set an unique identifier.
ERROR: 2016/08/11 09:09:48 site.go:1234: Two or more menu items have the same name/identifier in Menu "": "#IDCC16 Day 1: Open Data".
Rename or set an unique identifier.
ERROR: 2016/08/11 09:09:48 site.go:1234: Two or more menu items have the same name/identifier in Menu "": "#IDCC16 Day 1: Open Data".
Rename or set an unique identifier.
ERROR: 2016/08/11 09:09:48 site.go:1234: Two or more menu items have the same name/identifier in Menu "": "#IDCC16 Day 1: Open Data".
Rename or set an unique identifier.
ERROR: 2016/08/11 09:09:48 site.go:1234: Two or more menu items have the same name/identifier in Menu "": "#IDCC16 Day 1: Open Data".
Rename or set an unique identifier.
ERROR: 2016/08/11 09:09:48 site.go:1234: Two or more menu items have the same name/identifier in Menu "": "#IDCC16 Day 1: Open Data".
Rename or set an unique identifier.
ERROR: 2016/08/11 09:09:48 site.go:1234: Two or more menu items have the same name/identifier in Menu "": "#IDCC16 Day 1: Open Data".
Rename or set an unique identifier.
ERROR: 2016/08/11 09:09:48 site.go:1234: Two or more menu items have the same name/identifier in Menu "": "#IDCC16 Day 1: Open Data".
Rename or set an unique identifier.
ERROR: 2016/08/11 09:09:48 site.go:1234: Two or more menu items have the same name/identifier in Menu "": "#IDCC16 Day 1: Open Data".
Rename or set an unique identifier.
0 of 9 drafts rendered
0 future content
90 pages created
0 non-page files copied
43 paginator pages created
19 tags created
in 433 ms

My code is on github (branch “redux”), but I can pull together a minimal working example if that would help.

Ok, never mind — I’ve just realised that the version of Hugo on Ubuntu is out of date and installed the latest version from https://github.com/spf13/hugo/releases which seems to fix the problem.