You can avoid this error by ensuring that your subscript contains only unique values, using the MATLAB function unique. For example:
gvec = gpuArray(1:100);
% Indexes to assign - contains repeated indices
idxs = [1 1 2 3];
% Values to assign
vals = rand(1, 4);
% This would get an error - the assignment happens in parallel on the GPU,
% so which value should gvec(1) get - vals(1) or vals(2)?
%gvec(idxs) = vals;
% Use UNIQUE on idxs
[uniqueIdxs, locOfUniqueIdxs] = unique(idxs);
% Assign using only the unique indices, and the corresponding values
gvec(uniqueIdxs) = vals(locOfUniqueIdxs);
