Once an artifact is shared, it can not be shared again, which makes a lot of sense. However, once an artifact stack is shared, you can't share it again, even if you receive a new drop of the same artifact and add it to that stack. With friends looking for specific artifact shares, I often find I'm holding off on stacking my artifacts until my next "Share" opportunity.

When I stack new drops on existing artifact stacks, the share button should be re-enabled. Obviously if I stack artifacts and all instances have been shared the share button should remain disabled, but otherwise it should be enabled after stacking.