Jennifer Aniston and Courteney Cox have fans doing a double take.

In honor of her pal's 51st birthday, Cox took to Instagram to share a photo of herself with Aniston.

"No matter how hard you might try... there’s only one Jennifer Aniston. Happy birthday my dear friend @jenniferaniston! I love you!♥️♥️♥️" the 55-year-old star captioned the snapshot.

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The picture quickly caught the attention of social media users, as the "Friends" co-stars look nearly identical in the photo with both actresses wearing black tops, matching glasses, minimal makeup -- and finally, Cox appearing to rock a wig, that perfectly matched Aniston's hairstyle.

"TWINS," wrote one person. "

"Omg Twins," echoed another.

"Wait which one is Jen," asked one commenter.

Meanwhile, the pair's famous friends couldn't help but chime in, as well.

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"Hella cute," wrote Mindy Kaling.

"This is better than it should be. Good job," David Spade said.

"You both are the cutest #HBD @jenniferaniston," Rachel Zoe wrote along with heart and champagne emojis.

And of course, the birthday girl herself commented with a bunch of laughing, heart and kissing emojis.

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Aniston also took to her Instagram Story on Tuesday to thank everyone for all the sweet birthday wishes.