Very odd behaviour!
First of all, I have a site with a list of excerpts in home page. Every excerpt has a single thumbnail image. This is a native mechanism of WP, you just set for every article the image you want show.
Then, if I set to active the plugin AddThis WordPress show every thumbs DOUBLED! One a the side of the other one... I am not drunk, trust me :-)
I would investigate but I dont know if it is a bug already issued.
Any idea will be appreciated :)