Are there any downsides of having multiple versions of a website?

Hello;

A few years back I made some real life database driven website in order to learn php and mysql.

A few of those sites actually took off and started making me a little bit of money! Members started to join and now there are 10 thousands members! The way they've been coded looks absolutely ridiculous! Vunrable to sql injection, xss, no code structure,...

I started learning Codeigniter about a month ago and I can do things much better now. In order to learn CI, I started remaking those sites. Basically using the same database but proper.

I was wondering if there is any problems with having multiple versions of a website online? I will eventually remove the old sites but for now until the new ones pick up in google I am gonna have multiple ones. For example www.site1.com shows all the listings from my database and so does www.site2.com

So If anyone ads listings to this database, both these sites show it.

Is there any problems with that? I donno maybe Google doesn't like duplicates or any other issues I am not aware of?

It may even be a good idea to have multiple sites using the same database! Or is it?

Thank you.

This is not a PHP question, but an SEO one... Maybe a mod can move the topic.

Google doesn't like duplicates. If you have two sites that have the same information on them, Google will pick the one site to display in it's results and basically "forget" the other site.

Why not just update the first site with your new code and run it there instead of having two separate sites?

Moved.

Technically speaking there aren't really any problems with it. What is a problem is the SEO aspect - namely having duplicate content in different locations. Who is the source? Who is copying? The answers Google comes up with may cause problems for you.

Hello catacastic and requinix;
What about to those websites that have the same source of content? Maybe feed each other by XML but show exact same data from the database (news for example). Is that any different?

Thanks

Duplicate version of website is not a good idea. Till you must require this then use canonical tag in one of your website. Before use this tag R&D on canonical tag.

I'm not 100% sure but I think the canonical tag works only for content in the same website.
I don't think the source of your data matters to Google, what they finally see is the HTML version of your website. And in your case both have the same content.
I see a few problems here, Are you going to create a new site from and hope that thousands of members in the old site will just come to the new site ? I ask this because you have mentioned you will get rid of the old site. I don't think this is a good idea at all. Better to try to fix the old site.
If you want to test the code first maybe test it in a domain with no index or some sort of mechanism to block the search engine crawlers.

Even if they are partially similar like these two?
Link 1
Link 2

Duplicate content is duplicate content.

If you want a site to get into the search results, it needs it's own unique content. In an example of some news sites that get their data from a range of sources, the sites always point back to the actual source. I know that news.com.au has articles from all of the affiliated news sites appearing on it's main page, but the links go to the actual articles, not one that's been copied onto the main parent site.

In this case both content are in the same domain. So I'm sure you can use the canonical tag to get around this.