Double Tall Grass is not affected by fortune, as shown in the attached video by only one seed dropping. However, regular grass drops 5 and 7 in the video.

Steps to reproduce:

1. Get a tool with fortune 3.
2. Mine tall grass, and count how many seeds are gathered.
3. Mine normal grass, and count how many seeds are gathered.
4. Note that the normal grass will give more seeds.

Expected result:

Double grass should either drop the same amount of seeds, or double.

Link for reference —> https://youtu.be/I0dZ_arvjW0