How to draw a double empty ASCII rhombus?

How to draw a double empty rhombus with asterisks? I have implemented the rhombus itself, but I don’t understand how to make it double.

     *          *
    * *        * *
   *   *      *   *
  *     *    *     *
 *       *  *       *
*         **         *
 *       *  *       *
  *     *    *     *
   *   *      *   *
    * *        * *
     *          *

My code:

int i, j, n = 6;
String str = "";
for (i = 1; i <= n; i++) {
    for (j = n; j > i; j--)
        str += " ";
    str += "*";
    for (j = 1; j < (i - 1) * 2; j++)
        str += " ";
    if (i == 1)
        str += ("n");
    else
        str += ("*n");
}
for (i = 2; i <= n; i++) {
    for (j = 1; j < i; j++)
        str += " ";
    for (j = 1; j <= n * 2 - (2 * i - 1); j++)
        if (i == 0 || j == 1 || j == n * 2 - (2 * i - 1))
            str += "*";
        else
            str += " ";
    str += "n";
}
System.out.println(str);

Answer

Imagine a coordinate plane and draw a graph of the function in a rhombus form:

Math.abs(y - n) + Math.abs(x - n) == n

Try it online!

The zero point is in the upper left corner, and axes are directed downward and to the right:

// size of the rhombus
int n = 6;
// number of rhombuses
int m = 4;
// vertical axis
for (int y = 0; y <= 2 * n; y++) {
    // horizontal axis
    for (int x = 0; x < 2 * n * m + m; x++) {
        // current rhombus
        int k = x / (2 * n + 1);
        // center of the rhombus
        int c = (2 * k + 1) * n + k;
        // check if this point is on the rhombus
        if (Math.abs(y - n) + Math.abs(x - c) == n)
            System.out.print("*");
        else
            System.out.print(" ");
    }
    System.out.println();
}

Output:

      *            *            *            *      
     * *          * *          * *          * *     
    *   *        *   *        *   *        *   *    
   *     *      *     *      *     *      *     *   
  *       *    *       *    *       *    *       *  
 *         *  *         *  *         *  *         * 
*           **           **           **           *
 *         *  *         *  *         *  *         * 
  *       *    *       *    *       *    *       *  
   *     *      *     *      *     *      *     *   
    *   *        *   *        *   *        *   *    
     * *          * *          * *          * *     
      *            *            *            *      

See also: Output an ASCII diamond shape using loops