I was wondering if there are problems with putting share buttons at the top and the bottom of the post. Reason is that some people share on the top, and others at the bottom.
Reading your comment above, I believe that as a reader, you should only share something that you have fully considered the value of and have concluded that it is of interest to others and worth sharing.
So, putting share buttons at the top
However, that's not to say you should not actually put them at the top!
If the layout of the page is such that you can put the social buttons more discretely (smaller, less text) at the top, and more prominently at the bottom of the blog article (larger, 'Share this page' for example), then that's a great balance for many situations. Plenty of popular high profile blog/news sites do exactly this.
In your mind, understand the way in which top-of-page sharing and bottom-of-page sharing actually gets used. Include it for the right reasons and to promote responsible sharing, and the design should follow.
On a side-note - if the articles are long, include a discrete back-to-top link at the bottom of the article, and the comments - for example as seen on Smashing Magazine
You should be able to easily count the relative usage of top and bottom sharing, and you can A/B test with/without the back-to-top link and see the difference in whether more people then use the top share after clicking it. Use real data to affect ongoing design changes.
You can also consider having the share buttons float along the side of the article so that it never leaves the user's view. That way, they can share at the beginning if they find it valuable "at first sight," somewhere in the middle or at the end. I like Mashable's implementation:
See any article for reference: http://mashable.com/2012/05/18/facebook-ipo-closing-day-price/
Who says you have to make a choice? Many blogs and news sites these days have 'sticky' social media toolbars that stay in fixed position as the user scrolls the document. These allow users to always discover the social buttons, and to share the document as soon as it proves interesting.
If that's not possible, then it depends on how you expect your content to be consumed. Are your users only going to qualify that something is share-worthy once they've read the post? Stick it at the bottom. Is there content at the top which immediately qualifies the article as worthy of sharing? Upper controls may be better. Consider performing an A/B test, trying out both variations and tracking the number of shares, to find the best choice for your circumstances.