Does Essence Drain stack?

If I hit enemy with ED twice, does it now have one or two stacks of it?

Debuffs with the same name generally do not stack unless they specifically state otherwise.

The debuffs are applied to the enemy, but only the strongest one takes effect. If the debuff stacks but not infinitely (except for technical limits), the strongest ones up to that limit take effect.

This means that weaker debuffs with a longer duration can keep applying their effect when a shorter stronger debuff expires even if it was applied later. For Essence Drain this usually does not matter, since neither your duration nor damage should vary considerably between two casts.

Yeah, seems simple question but I somehow couldn't find clear answer, thanks.

So basically, you pop it once (bane build) on buffy targets and bosses every few seconds. I was shooting it nonstop like plasma pistol.

The advantage that Essence Drain has over Blight and Soulrend is that spamming is not necessary, which gives you more time to dodge boss mechanics. The theoretical single target damage of Blight and Soulrend is higher, but you usually do not have as much uptime. Getting close to 100% uptime on Essence Drain is easy.

While Blight stacks up to 20 times, Soulrend just has a low enough duration that you need to keep casting it regularly.

If you want to play Blight or Soulrend against single targets, you need to invest more into defence in order to ensure your survival while you stand still.