Goosebumps-like Teen Alternate Universe Story

[nalkarj]

If no one minds, here’s another one that I’ve been completely unable to find over the last few months; this one particularly annoys me because I thought it would have been an easy question! :)

Here’s what I wrote:

“I'm sure this is an easy one, but I don't exactly have many leads, so…

“I remember reading (at Borders!--how I miss those stores) a Goosebumps-like book—possibly also by R.L. Stine—in which a young boy, the narrator, is transported to an alternate universe that is exactly like ours except for a few minor alterations (e.g., he takes karate lessons in the alternate world--I'm fairly sure I remember that), so he doesn't realize that it's an alternate world at first.

“At the end, he has to confront his own alternate-universe counterpart, and (here's the requisite Goosebumps-style twist) he finds out that he can't actually leave the universe, or something like that. Cue horror for the narrator. I know for a fact it's not Stine's Don't Go to Sleep!, which also involves alternate universes, and I don't think it's a Goosebumps book, merely something similar.

“Thanks!” 

Now, for all I know, it could have been a Goosebumps book and I was just misremembering, as I’ve been utterly unable to find it. Thank you all very much.