They were sent by someone you already met.
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This person made clones of your party before.
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He made clones of others in the past. You found one.
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He made the first batch of clones of your party in Spellhold, where you defeated him for the first time, after escaping his maze. You discovered him cloning an elf lady while escaping from his dungeon.
I bet I don't need to spell this out further
-- Nifft, or perhaps Nifft's evil clone