It doesn't happen to me either:
int a = 1, b = 2;
if(a == b)
{
b = a;
}
if(a == b)
{
b = a;
}
If I then reformat using CTRL+K,D I get a better looking version:
int a = 1, b = 2;
if(a == b)
{
b = a;
}
if(a == b)
{
b = a;
}
but the line doesn't vanish. And I haven't turned on any options like that, most of my formatting options are defaulted.
Have you perhaps installed a "code style" extension which is doing it? I believe resharper can ...