301 redirect in IIS

So we have a client whereby we are hosting their site but with 2 different domains mapped (we have bindings for both domains) to the same site in IIS. this is primarily because when they started with us they could only purchase the domain they didn't ultimately want to use going forwards.

For example the domains are www.some-thing.com and www.something.com.

They have asked if I can put a 301 permanent redirect in on the URL with the hyphen so then users should always go to the non hyphen version of the site.

I have tried URL rewrite rules and using the HTTP redirect in IIS however the site is then in a re-direct loop and I think this is down to the fact I have both domains bound to the site in IIS.

Help !!

Thanks in advance