Block stack executes when duplicated blocks are attached

[carljbowman]

Expected Behavior

Attaching duplicated blocks to a stack, should not cause the stack to run.

Actual Behavior

Attaching a duplicated block or a duplicated stack of blocks to another stack, will cause the receiving stack to run.

Steps to Reproduce

1. Create a stack of blocks, preferably something that will take some time to execute so you can see the result.
2. Drag another block on to the workspace, but do not attach it to the stack from the first step
3. Right click and duplicate the block from step 2 and attach it anywhere to the stack from the first step