Add parameter allow_address_duplication

[Koc]

Hello!

In my env single ip has many hosts. I cann't use notation like

172.18.0.2 site1 site2 site3 site4 site5

because site4 and site5 not resolves on Windows (looks like OS bug).

So I should write hosts in another way:

172.18.0.2 site1
172.18.0.2 site2
172.18.0.2 site3
172.18.0.2 site4
172.18.0.2 site5

But this is impossible for now. Adding new parameter allow this.

[jonhadfield]

Thanks @Koc. Merged and published as version 0.4.1.