Here's a method that works for what you want to do. If X is length n x 1, create the modifier such that it is only n/2 x 1. Then, multiply the modifier by [ 1 1 ] a 1 x 2 array. This results in an n/2 x 2 array. Then, reshape the transpose to create an n x 1 array which will have each element repeated once. Here's the code:
modifier = percent(rand_percent(1:length(X)/2));
modifier = modifier * [ 1 1 ];
modifier = reshape(modifier',[],1);
Then, the final result can be multiplied element-wise by X:
Y = X.*modifier;
