Jennifer Aniston, Courteney Cox stun fans in Instagram birthday post: 'Twins!'

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.

JENNIFER ANISTON TURNS 51: A LOOK BACK AT HER BIGGEST MOMENTS

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.

BRAD PITT, JENNIFER ANISTON RUN INTO EACH OTHER AGAIN AT OSCARS AFTER-PARTY: REPORT

"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.

CLICK HERE TO GET THE FOX NEWS APP

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