Tips & Tricks
The Doppelgangers use Cael Hammers exclusively, so using long ranged weapons and keeping your distance are ideal tactics to use against them.
If you are fighting them after falling unconcious in Jawson's Bog then it is quite easy to just back off while firing a ranged weapon, rolling in order to stay out of hammer range.
In this way, you can destroy the Doppleganger while it's still in Cael-Hammer form with a weapon intended for long-range attack, like the Galleon Mortar.
If you are in the Stockpot, there are three on the last level. Just stay agile, use the skybridges, roll often and counter as much as possible.