Yes, you may have the same image on all the micro stock sites. (That is a good thing about royalty free images)
One exception however, is if you decide to list an image as 'exclusive' on a given site. Certain sites, such as dreamstime, canstock, and fotolia (amoungst others) offer this. That means you promise THAT particular site, that they have the exclusive right to sell the image. They are the only ones allowed to sell the image.
Another exception is if you list yourself as an exclusive PHOTOGRAPHER to one site, such as istockphoto. Then you may ONLY sell photos on Istockphoto. (not a good idea in my opinion)