Quote Originally Posted by Flickerdart View Post
Yeah, that's what I think he means - you can only catch something in the first encounter on each route, unless it's a species you already have (species clause). So you have to keep track of which routes are "dead" to you, because you messed up and knocked that Pokemon out.
I find it not to be difficult, because the question is effectively the same as "have I been through this route yet?" Occasional exceptions where there's one big route with enough distinct parts that you lose track, or when route boundaries are not clear.