Valmiki Ramayana Yuddha Kanda Sarga 110 Shloka 28
Original Shlokaद्वितीयंरावणशिरश्छिन्नंसम्यतिसायकैः । छिन्नमात्रं च तच्चीर्षंपुनरेवप्रदृश्यते ।।6.110.28।।
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सम्यति before that, द्वितीयम् second, रावणशिरः Ravana's head, सायकैः by arrows, छिन्नम् cut off, तत् that, शीर्षम् head, छिन्नमात्रम् as he cut, पुनरेव again, दृश्यते appeared
Before cutting that second head of Ravana again another head appeared.