The generator matrix

 1  1  1  1  1  1  1  1  1  1  1  1  1  X  1  X  1  1  1  1
 0  X  0  0  0  0  0  X  0  0  0  0  X  X  X  X  X  X  X  X
 0  0  X  0  0  0  0  X  0  0  X  X  X  X  X  0  X  0  0  0
 0  0  0  X  0  0  0  X  0  X  X  0  0  0  X  X  X  X  0  0
 0  0  0  0  X  0  0  X  0  X  0  X  0  X  X  0  0  X  X  0
 0  0  0  0  0  X  0  X  X  0  0  X  0  0  X  X  X  0  X  0
 0  0  0  0  0  0  X  X  X  0  X  0  0  X  0  0  X  X  X  0

generates a code of length 20 over Z2[X]/(X^2) who´s minimum homogenous weight is 16.

Homogenous weight enumerator: w(x)=1x^0+57x^16+32x^18+85x^20+32x^22+38x^24+10x^28+1x^36

The gray image is a linear code over GF(2) with n=40, k=8 and d=16.
As d=16 is an upper bound for linear (40,8,2)-codes, this code is optimal over Z2[X]/(X^2) for dimension 8.
This code was found by Heurico 1.16 in 6.29e-008 seconds.