In the current version of the strengthen mode for this sentence, the only answer accepted is 'next', while the answer presented as 'correct' has 'adjacent'. Perhaps 'adjacent' should be accepted for all uses of διπλανό, since it seems to have the most general meaning, 'next' and 'adjoining' imposing some additional requirements
I believe 'adjacent' as opposed to 'next' would feel more natural in English and be a more accurate translation of the Greek.
The word "next" concerns two things, next in space and next in time. In case of διπλανός/ή/ό it is next in space. So adjacent is the answer. Definitely in space. If we want to use next in time we can use "επόμενος/η/ο", i.e. επόμενη (χρονική) στιγμή=next moment. But never διπλανή (χρονική) στιγμή. The () in this case can be omitted, most likely since we are talking about time. Another example: επόμενη μέρα=next day, but never διπλανή μέρα. Or just simply, αύριο in case we are talking about today.
Okay. In that case I would consider the English sentence to be incorrect as "next" as a standalone adjective almost exclusively refers to the temporal sense (the major exception being "next door", but even then it's a figurative construction.) Reading "The next chair is dirty" I would think something like, the speaker is picking chairs, and goes with this one because the next chair (the chair they would go to next) is dirty. For what the Greek sentence seems to be indicating I'd probably instead say "The adjacent chair" or "The chair next to this one" or "The other chair". In other contexts perhaps, like "So-and-so was sitting in the next chair", but even then "the next seat" sounds more natural.
I looked at the proposed answer for my post about "next: "Next chair". I didn't examined next as a standalone adjective. No objections about the several use of next. But in Greek διπλανός is spacial. In wordreference one can find several translations. In this very case, if we want to be precise and correct I don't think we can use the translation κοντινός or γειτονικός, even sometimes it is used. It is a bit fuzzy and not clear the distance for each one of us :) http://www.wordreference.com/gren/%CE%B4%CE%B9%CF%80%CE%BB%CE%B1%CE%BD%CF%8C%CF%82
About subsequent here is a translation from the same dictionary. It is temporal: http://www.wordreference.com/engr/subsequent