Vyxal j, 10 bytes
kA:ǓZv∑13*
kA # Uppercase alphabet
:ǓZ # Zip with a copy rotated one to the left
v∑ # Sum each
13* # Repeat each 13x
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Visit Stack Exchangej, 10 byteskA:ǓZv∑13*
kA # Uppercase alphabet
:ǓZ # Zip with a copy rotated one to the left
v∑ # Sum each
13* # Repeat each 13x