The adjectives “isti” and “jednak” in Standard Croatian do not have the same meaning. The adjective “isti” signifies “the one that was before”, and the adjective “jednak” signifies “the one that is not different from the one it is being compared to”.
If we say: „Vidjeli smo istog čovjeka prošli tjedan.”, it means that we saw a man who we also saw last week, i.e. it is the same man. If we say: „Vidjeli smo jednakog čovjeka prošli tjedan.”, it means that last week we saw a man who looks the same as the man we saw today, but they are two different men.