Duplicated additional hostname breaks ddev start

[tmotyl]

What happened (or feature request):

I have added an additonal hostname with the same name as project name. This prevented ddev from starting.

What you expected to happen:

I would expect ddev to ignored duplicated name, or fail with a meaningful error saying what is wrong.

How to reproduce this:

copy project name (name attribute value) to additional_hostnames
call ddev start

[rfay]

This is definitely true, is mentioned in the docs I think but it's worth solving and not too hard.

Also, of course, the name can't be duplicated among projects, that's a harder problem to solve.

[rfay]

What you see when this happens:

DDEV ROUTER STATUS: unhealthy
The router is not currently running. Your sites are likely inaccessible at this time.
Try running 'ddev start' on a site to recreate the router.

docker logs ddev-router shows

"GET /healthcheck HTTP/1.1" 404 169 "-" "curl/7.52.1" "-" nginx.1 | 2018/04/30 13:43:36 [error] 35#35: *233 open() "/usr/share/nginx/html/healthcheck" failed (2: No such file or directory), client: 127.0.0.1, server: localhost, request: "GET /healthcheck HTTP/1.1", host: "localhost"