Domain name "hijack" and Google penalization

Hello all

Recently I have discovered that there is another domain pointing to my website.
To be more clear, checking my keywords on Google I have found some links from another site let's call him www.theother-site.com, following the links, I landed on my website, I was browing my website on a different domain name!
Basically if you type in the browser: www.mywebsite.com/index.php or www.theother-site.com/index.php is exactly the same, you are landing on my website. Is not a redirect because it maintain the different domain name, it not actually redirect on my website domain name.
How is possible that this happens, and can this cause Google thinking that I am duplicating my site on 2 different Domain with the consequential penalization?

Thnx

 

Don't worry about it. There's no penalty for duplicate content. Read this: http://googlewebmastercentral.blogspot.com/2006/12/deftly-dealing-with-duplicate-content.html

 

Thank you qazu for your answer.

But I am still a bit perplexed that having 2 different domain pointing to the same website is not seen as a duplicate content bt Google.

 

It seems that somebody redirected his domain to your site.

 

Hi johankrava and thank you to have jumped in,

Sorry I am not a DNS expert, you mean that somebody pointed their Domain Name NameServers to my Hosting Provider NameServers to redirect his domain to my site,or they use some sort of Domain Masking?