Add an array with itself

Python 3, 67 65 42 bytes

Uses the fact that the sum of an empty array is 0

lambda x,y:[sum(x[i::y])for i in range(y)]

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MATL, 4 bytes

e!Xs

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First bit of MATL code I've written! Takes two inputs, a as a row vector (comma-separated) and l as a number. Works out to be

e          # reshape `a` into `l` rows (auto pads with 0)
 !         # transpose
  Xs       # sum down the columns

Jelly, 7 6 bytes

1 byte thanks to Dennis.

;0$¡sS

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