I'm planning to replace my stock head unit that is single din
to same year stock head unit that is (6 CD) double din in my Impreza.
Do I need a different mounting bracket for double din unit?
And will I need harness adapter?

Nope it should just bolt and plug right in. You use the existing brackets, instead of mounting the single DIN unit plus the blank plate or pocket you'll be mounting the dual DIN to the same brackets.

Nope it should just bolt and plug right in. You use the existing brackets, instead of mounting the single DIN unit plus the blank plate or pocket you'll be mounting the dual DIN to the same brackets.
That's what I was hoping to hear, thanks a bunch!