Me and a friend were playing one day and then he put down Alter Ego and copied my Hamletback Goliath. I couldn’t find any thread or anywhere on the wiki saying what to do in this situation. We ended up just saying that Hamletback and Alter Ego we’re both infinite/infinite but that Alter Ego could destroy Hamletback in combat. I’m not sure if there is a better way to do this and I’m curious to know what other people’s thoughts are on the subject.
I am not sure what you are thinking will happen here. The Ego enters as a 6/6 Goliath with as many counters as was chosen for X. This causes their Goliath to trigger to get more counters. The process ends there. So, if the original Goliath had 3 counters already, and X was set to 2, the Ego/Goliath enters with 2 counters so it is an 8/8. The original Goliath gets 8 more counters which puts it up to 11 counters. Now, there is the original Goliath which is a 17/17 due to the counters and the new one which is still an 8/8.
Counters are not copiable so the Ego won't get any of the counters the original had and it doesn't keep changing if the source keeps changing. It takes a "snapshot" of what it is copying and it won't change from that if the original changes after.
Also, as a side note, there is never "infinite" in Magic. There is always a number.