Jenna Fischer's standing behind her British doppelganger ... showing support for her 'Office' inspiration Lucy Davis after the Brit revealed she's dying of incurable cancer.

Jenna shared a kind comment on Lucy's cancer announcement post ... telling Lucy she loves her and saying it's important to normalize working while battling a disease, because it can provide comfort to some.

As you may know ... Jenna's beloved character Pam in the U.S. version of "The Office" is the American equivalent of Dawn Tinsley -- Lucy's character in the OG British show.

On top of understanding Lucy on a professional level, Jenna also has her own experience with breast cancer ... remember, she revealed in 2024 she'd been diagnosed with triple-positive breast cancer.

Difference is ... Jenna beat cancer after going through surgery, chemo, and radiation ... and Lucy says her cancer is "incurable."

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While Jenna was diagnosed with stage 1 cancer, Lucy's disease is more advanced. She said she has stage 4 breast cancer, which has metastasized to her bones.

Lucy says she plans to make the most of the time she has left ... including committing her time and energy to helping rescue animals via her charitable efforts.