My WASR Ak has a Tapco G2 set. I find it annoying that when I bench fire the rifle to zero it can occasionally double. It never does it when shoulder fired, so I think in a benched position I'm probably causing it to inadvertently bump fire. How do I fix this without buying a new trigger kit?

Either single load, or back off a touch on your trigger overtravel stop.  Reset travel is too short right now.

I don't believe a G2 has any adjustments. Simply hold the trigger back during the shot - its called 'follow-through' and will also improve your accuracy. Some G2s have a very light, smooth pull and you have to be mindful of them. I have a SIG 556R with a hard trigger. If I go from the SIG to my AK with G2, I have to conciously think about about keeping my finger out of the trigger guard until ready to shoot because its so light compared to the SIG.

I had the same thing happen to me when I was prone zeroing my rifle. You are bump firing it by accident. I just made sure I held the rifle tight in my shoulder and it stopped.

Does a weak hammer spring contribute to bump firing?

No. If anything it'd contribute to a FTF.

The G2 is fundamentally defective in design.  That it works ok in many guns is more  a credit to the gun, then this trigger group.

The geometry of the hammer is such that it isn't pushed back far enough to catch the disconnector in some guns.  It is especially bad in US receivers with a low trigger hole, such as the NoDak 3 Economy.  But it also happens in factory receivers.

Those with good trigger control experience it more than those who mash the trigger to the rear.  It is holding the trigger just at the point of release, that exacerbates the issues with the G2 hammer/disconnector relationship.

Solution is pretty easy.  Lower the rear shelf on the hammer by .020" (thickness of a box cutter blade).  Then file the front underside of the disconnector so at the rest position is almost touching the back of the retracted hammer.

AK trigger groups are fitted to the gun.  That's why they are numbered.  Never assume an AK trigger is "drop in".  Many are, in many guns.  But not always, and not with all shooters.

Is there an aftermarket trigger that will drop in without modifications, or should I just rework the G2 and call it good. I think my G2 trigger is too light and overtravel is ridiculous. I only get doubling/tripling when shooting off sandbags, never off hand or kneeling.

I was thinking stripping all lube from the trigger might help?

It's not possible to keep lube off trigger parts.  
Normal operation of the rifle will spray lube around and coat the parts.

Years ago, some crappy 1911 gunsmiths would botch the trigger job and they'd get hammer follow.
They basically BS-ed the owner and would tell him their trigger jobs needed to be run dry.
Fire a mag or two and the trigger parts had lube on them.

I had the same problem with doubling my AK-74 off the bench.  I was doing the normal careful trigger control and the the rifle was bump firing off my trigger finger.
You can also get doubling with other rifles like the M1 Garand off the bench.

The fix for the AK was to stone the lower-front of the disconnecter to move the top of the disconnect toward the hammer.
With more of a "bite" on the hammer the doubling was cured.

Over travel is a "feature" of the AK trigger assembly.  It was never intended to be a Match rifle or fired off a bench much.
If you insist on trying to make it something it's not, there are expensive adjustable triggers made for the AK.

I also had a light trigger and soon after I got the rifle it started mis-firing.
I found that the "legs" of the hammer spring were too straight and there wasn't enough hammer power.
I put more of a downward curve in the spring legs and that cured the misfires, and gave a heavier trigger pull, which it needed.

Most aftermarket triggers are just as light, if not lighter.

pull out the carrier.  push the hammer back.  
The back of the hammer should almost touch the front of the disconnector.
 Remove material from the underside front half of the disconnector, until this gap should be as close to zero as possible.