There is a legend that the Sita that Demon Ravana kidnapped from the forest was not the real Sita but Mayasita – one that looked like Mata Sita. This interesting story is found in the Devi Bhagavatham but is not mentioned in the Valmiki Ramayan. One day when Sri Ram, Lakshman and Mata Sita were on exile in the forest, Agni Bhagavan – the Hindu Fire God – appeared before Sri Ram and reminded him that objective of his avatar was to kill Demon Ravana and the time has arrived for Ravan to kidnap Mata Sita.
Agni thought that it was improper for demon Ravana to touch Mata Sita, an incarnation of Goddess Lakshmi. Therefore Agni suggested that He will guard the original Sita and provide a Maya Sita to Sri Ram.
Sri Ram agreed and the original Sita was thus replaced with Mayasita.
Ravana thus kidnapped Mayasita from the forest.
After Ravana was killed, Lord Ram asked Mayasita to undergo Agnipariksha to prove her chastity. When Mayasita entered Agni (fire), Agni Bhagavan took her back and returned the original Mata Sita to Sri Ram.