I thought mixins didn't override?

[Engine Machine via Digitalmars-d-learn]

I try to use a mixin template and redefine some behaviors but D 
includes both and then I get ambiguity. I was sure I read 
somewhere that when one uses mixin template it won't include what 
is already there?

mixin template X
{
   void foo() { }
}
struct s
{
   mixin template
   void foo() { }
}

I was pretty sure I read somewhere that D would not include the 
foo from the template since it already exists.