I want to subscript with characters to make output of a matrix, and followings can be done

    Subscript[a, bb] // TraditionalForm
    Subscript[a, b,b] // TraditionalForm
    Subscript[Subscript[a, b], b] // TraditionalForm

The results are following
$$
a_{bb}\\
a_{b,b}\\
a_{b_b}
$$
I want the first format.

But, I need to subscript $a$ with elements inside a list, so,

    list = {b,c,d};
    Subscript[a, list[[1]] list[[1]]] // TraditionalForm
    Subscript[a, list[[1]], list[[1]]] // TraditionalForm
    Subscript[Subscript[a, list[[1]]], list[[1]]] // TraditionalForm

Then, the result is
$$
a_{b^2}\\
a_{b,b}\\
a_{b_b}
$$

I don't want the second or the third. So, How do I prevent that $b^2$? Or, how do I erase comma from second code?