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Charlie Puth and Brooke Sansone already decided to Marvin Gaye and get it on ... and, we got pics of them hanging out hours after they announced the result -- a baby on the way!

Charlie wore a business casual ensemble ... a button-down shirt and work jeans -- with an untied tie wrapped around his neck. Backgrid

Brooke looked like a vision in a tight white shirt which barely contained her baby bump under a black leather jacket. Uncelar exactly what they're doing here, but they seem content just to be spending time together. Backgrid

There's no surprise these two look so happy -- 'cause they've got a little one on the way they just can't wait to meet!

As we told you, CP announced the news in the music video for "Changes" ... during which

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