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We are partying with Britney Spears and Melissa Joan Hart like it's 1999 -- because they have REUNITED. 

Melissa attended Britney's "Piece Of Me" show in Las Vegas on Wednesday night -- almost 16 years after the pop princess recorded the theme song for her 1999 romantic comedy Drive Me Crazy.

Britney and Melissa were all smiles posing together backstage. The singer thanked her for coming to see her perform, tweeting that it was "so good to see [her]" and that it had been "TOO long."  

SO. MANY. FEELS.

Remember when MJH made an appearance in Brit's "(You Drive Me) Crazy" music video?

And then Britney appeared on "Sabrina The Teenage Witch?"