Excel 2007 : choose common text from different cell with Multiple text

I want to make comparison between multiple texts of two different cells and want to choose common text

For example - excel cell A3 - James,Peter, Robert and excel cell A4 - Peter, Tom, Henry then ecxel cell A5 should give answer Peter(common word between A3 and A4)

Can you help me to put formula or program ??

Mayank,

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