Are the imperative and infinitive always completely reasonable alternatives?
Сидеть! - сиди!
не повторя́ть! - не повторя́й!
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There is a slight difference between them, the infinite sentences used as imperative are much more aggressive and exigent. For example, only infinite sentences are used to train dogs:
Ша́рик, сиде́ть! Sharik (dog's name), sit!
Ты посиди́ здесь, а я пойду́ посмотрю́, в чём там де́ло. Stay (sit) here, and I will see what is happening.