Google penalty?

I have a site with 10k posts. 2-3 weeks ago there were 300-400 pages indexed. Then added like 100-200 similar posts

Example:
site.com/travel-to-paris
site.com/paris-pictures
site.com/holidays-in-paris
...
site.com/holidays-in-italy
..

They have different images but the title is similar

Now many of these sites are indexed but my other posts were desindexed. Now only have 200-300 pages indexed.

Is this a penalty?
What can I do? Delete similar posts (travel-to-paris, paris-pictures, etc) And reupload sitemap.xml

Regards

Add some more new content that is UNIQUE in those categories to get SE indexing...

Sounds like what you added was too similar which the robot has viewed as copy n paste aka duplicate I would edit or delete that lot...

add more content man!

Thanks.

I am adding more content and deleting the similar post, but how tell google I am deleting the post? only updating the sitemap.xml?

I do not want to generate a lot of 404 errors.

Thanks for your help

Set up your site with Google's webmaster tools. If there are crawl issues with your site they should be reported there. It will sometimes let you know if you are suffering from an unnatural link penalty. Not sure about penalties related to duplicate content but if you have a lot of dup content then you are almost always going to be penalized.

Dupe content posts are fine as long as the titles and meta are different, if not then they will get deindexed.