The W or "duplicate" function in Action and Batch is awesome. I didn't know until three minutes ago that you could use it in Batch. Doesn't work on complicated nodes like action, but it will dupe 2d transforms, color corrects and logic ops.
All it does is make two nodes identical, so if you change one of them, the other changes too. It doesn't matter which way the green arrow is pointing, but you can't have a duplicate slave drive another duplicate--all the green arrows have to come from one source.
Beyond the general awesomness of this (when i have more time I'll post examples of why this is a handy feature) you can also set up one node like you like, then W-duplicate all the others, then sever the connections so you can modify each one off of the original.