A quick question for you.
Did you replace just one injector or all eight.
I work on 6.6 everyday and what is scary is that I, like you, have replaced one injector and then another went out. Do you have the ability to check codes and see if there is a code in the computer. If you can I can give you the specs and see how close if any are out of specs. If the same one is bad again then we can address that.
I have seen after installing a injector that it has been bad. I have seen while removing the old injector people have left the copper compression ring in the bottom of the injector sleeve. But the only way to get a fuel knock again on the same clyinder is because that injector is dumping fuel in the cylinder. I think the best thing to do is see if you know anyone with a snap on scanner or a gm scan tool to see the specs. Please let me know what you find