I need a 6mm packer piece for a suspension bracket. Is there any issue with welding 2 3mm sections that I have in the scrap pot together. I have been told that a weld joint is stronger than the metal itself therefore I don't see it being a problem but given its use want to check before I weld into position.

Should be fine - put a bit weld prep on both bits and then it certainly will be.

Depends how its going to be applied

You might even drill a ten mm hole in the middle of one piece then plug weld it to the other after tacking them together. Or drill through both then plug weld to chassis.

Indeed a good weld is stronger than the original material(also more brittle)(unless the original material was heat treated)
But the material around it (HAZ) has been weakend.

All depends on your intended loads and how they will be applied.

Even the exhaust manifolds on a Spitfire are double skinned so you wont have a problem. If you were building to Aircraft spec you would have to drill a hole through one piece to allow gases to escape whilst you were welding. The gap between to two pieces would also be vunerable to water ingress and unseen corrosion so make sure it is well sealed.....