Zhè jiàn shì qiānzhēn-wànquè, wǒ qīnyǎn kàndào de.
This is absolutely true - I saw it with my own eyes.
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grammar
千真万确 follows the 千X万Y pattern common in Chinese idioms: 千变万化 (ever-changing), 千辛万苦 (untold hardships), 千言万语 (a thousand words). The 千/万 pairing emphasizes extreme quantity.