I was trying my hands on .NET 3.5, and discover that Gridview was firing Rowcommand twice for my ImageButton that is draw using ButtonField Column. Later I found that this is  a bug/unexpected behaviour from out of the box Gridview. I found some solution on internet that can be seens in detail here:

http://forums.asp.net/p/1002747/1324414.aspx#1324414

It mainly suggest to use TemplateField column with imagebutton, as Imagebutton is not quite compatible with ButtonField. Rest you can read above for details on it.