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Courteney Cox is practically Jennifer Aniston's twin in birthday tribute

Cox is celebrating her "dear friend" on Aniston's special day.
/ Source: TODAY

Jennifer Aniston turns 51 on Tuesday, and her friends wanted to make sure that she feels plenty of birthday love.

Courteney Cox led the charge on Instagram by posting a photo of her hanging out with her "Friends" co-star and BFF.

"No matter how hard you might try... there's only one Jennifer Aniston," Cox wrote. "Happy birthday my dear friend @jenniferaniston! I love you!"

Fellow "Friends" cast member Matthew Perry, who joined Instagram just last week, posted a throwback pic of himself with Aniston in honor of her special day.

"Happy birthday, Jenny!!!" he captioned the photo.

Aniston's ex-husband, Justin Theroux, also wished her a happy birthday on Instagram.

"Grabbing 2020 & another year like," he captioned a picture of the actress shouting with her fists clenched. "Happy Birthday B."

Aniston's ex, Justin Theroux, posted this Instagram story in honor of her 51st birthday.
Aniston's ex, Justin Theroux, posted this Instagram story in honor of her 51st birthday.@JustinTheroux/Instagram

It's not clear what the "B" nickname means — it's definitely not her middle name, which is Joanna.

Reese Witherspoon, who loves to shower her friends with birthday wishes on Instagram, shared a photo of her and Aniston smiling for a photo together at the 2017 Vanity Fair Oscar party.

"Happy Birthday Jen!" Witherspoon wrote on Instagram. "I couldn't ask for a better friend and partner to chase my dreams with. Life is way more fun with you in it!!"

While Witherspoon and Aniston are earning rave reviews for their performances on "The Morning Show," the pair go way back. Witherspoon guest-starred on "Friends" during the 1999-2000 season as Jill Green, the little sister of Aniston's character, Rachel Green.

Jennifer Aniston and Reese Witherspoon on "Friends"
Reese Witherspoon guest-starred as Jennifer Aniston's little sister on "Friends" during the 1999-2000 season.Getty Images

This is the first birthday Aniston has celebrated as an Instagram user after joining last October. Aniston's first post was a selfie of her hanging out with her former "Friends" co-stars. It racked up more than 15.8 million likes and led to fans, including actor Orlando Bloom, expressing the way we feel: "Finally."

Aniston now has 27.6 million followers on Instagram and has delighted her fans by posting plenty of hilarious throwback pics and fun glimpses into her everyday life.

Happy birthday, Jen!