www.alphadictionary.com

Normally, when we add -st to a word other than a verb, it is just a variant, and the meaning stays the same, e.g:

while/whilst
among/amongst
unbeknown/unbeknownst

However, again and against are two distinct words, with no obvious semantic connection. Are they etymologically related?

One interesting point here is the similarity of the German words wider, 'against' (a formal/literary word), and wieder, 'again'.

An interesting question. It appears the answer is "yes." "against" seems to have started life as a variant of "again," just as you suggest. Etymonline's comment is here
https://www.etymonline.com/word/against ... sreference
I see the Good Doctor has not addressed either of these words, so I would like to encourage him to do both.

BJNTokyo, your wish is hereby granted: against will be the Good Word next Tuesday.