The host part is used both for matching and route generating (both by name), you can import an RouteCollection only once per host.

If you import the routes they inherit the configuration or there 'parent', if you import them again under a new host these will overwrite the ones previously registered (by there names).

If you really need to use multiple hostnames you can also change the host pattern to "^(my.\first.domain|any.other.domain)$"

Thanks for your reply. Unfortunately I cannot avoid importing the same RouteCollections for both domains, so I tried out the regex:

test:
    host:     "^(my.\first.domain|any.other.domain)$"
    resource: "routing.yml"

But like in #6848 this doesn't match any domain.

test:
    resource: "routing.yml"
    host:     "my.first.domain"
test2:
    resource: "routing.yml"
    host:     "any.other.domain"

The problem is that the routes are identified by name, so they are just overridden by the second import using host: "any.other.domain".

test:
    host:     "^(my.\first.domain|any.other.domain)$"
    resource: "routing.yml"

Sould be

test:
    host:     "{domain}.domain"
    resource: "routing.yml"
    requirements:
        domain: my\.first|any\.other

Please have a look at the routing documentation. Its the same style as for the path pattern.

Thanks for your suggestion, but using requirements would mean that I had to provide the domain parameter each time I generate a route like described in #6857. Using defaults wouldn't work for me as the user has to stay at the requested domain:

test:
    host:     "{domain}.domain"
    resource: "routing.yml"
    requirements:
        domain: my\.first|any\.other
    defaults:
        domain: my.first

With this config every generated route would point to my.first.domain even if the user requested any.other.domain.

Yes thats a known problem. See also #3965 that could solve it.
Closing as duplicate of #6857.

@tak-amboss As a workarround, as host don't support pattern you can use a condition:

test:
    condition: "request.headers.get('Host') matches '^(my.\first.domain|any.other.domain)$'"
    resource: "routing.yml"

But it's strange for me that pattern is supported by host declaration in firewall but not in routing.