In my opinion, this is just one more reason to make clan tags alphanumeric ONLY, like the names. This'll just happen more and more as people discover they can do it.
This isn't feasible. What if somebody had a clan tag of "e"? Then you couldn't use your name, since it starts with "e". If you're referring to just clan registration, then the check is already in there. If somebody registered [e], another clan couldn't register (e).
Sorry, I should have been more specific. This is what I'd like to see for clan tags:
Alphanumeric-ONLY clan name. This would be how each clan would be identified. For example, "i", "20ID", etc, etc.
'Padding'. This would be what 'separates' the tag from the name. No alphanumeric characters (perhaps even just a couple predefined characters you could choose from). When entering, an 'O' would indicate where the name would go. For example, "-O-", "(O)", ")O(", "]]O[[", etc.
Putting it all together, newbie's has the name 'omg' and padding '-]O[-'. His name in-game would be "-]omg[- newbie".
This system gives clans a decent amount of flexibility in how their tag appears without the confusion of (E) vs [E].
EDIT: BBcode screwed up my post. Had to make minor changes.