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Они меня с кем-то перепутали.

Наверно они приняли меня за идиота.

I wrote a reply but it was lost due to site crash. :(

(a) Normally "некоторых" = "some of ..." that is a selection from some specified set is implied. "Существует много книг по этому периоду. В...

I'd say just "Мне надо перевести документы других клиентов."

Basically you will be understood anyway. And all listed ways are quite acceptable. All the below you need only if you aspire for a literary ideal Russian, which will take from you many years (as you...