Replicating the bug:

1) Craft redstone lamp
2) Place redstone torch on floor
3) Place lamp on top of torch

One of two things can happen:

1) Lamp never leaves inventory yet gets placed anyway
2) Lamp leaves inventory

Now:

1) Destroy lamp

And watch as you suddenly have 2 redstone lamps. Rinse, repeat.

A bit of thinking seems to point towards an inventory related bug, since a lit redstone lamp has a different item ID, it seems to get placed and powered immediately, and the block update comes before the verification of the block being placed, so when the inventory is updated it attempts to take out a lit redstone lamp which you obviously don't have, and leaves the regular one alone.

Destroying the placed lamp then returns the lamp to your inventory, and since the original one never left, you now have two.