interp1(2:2:length(a)*2, a, 1:length(a)*2)

interp1(2:2:length(a)*2, a, 1:length(a)*2, 'linear', 'extrap')

interp1(2:2:length(a)*2, a, 2:length(a)*2)

a = 1:10;
t = [a(1:end-1);a(2:end];
t(2,:) = mean(t);

b = [t(:); a(end)]

>> a = 1:10;
>> b = interp1(a,linspace(0.5,numel(a),2*numel(a)),'linear','extrap')
b =
  Columns 1 through 8
    0.5000    1.0000    1.5000    2.0000    2.5000    3.0000    3.5000    4.0000
  Columns 9 through 16
    4.5000    5.0000    5.5000    6.0000    6.5000    7.0000    7.5000    8.0000
  Columns 17 through 20
    8.5000    9.0000    9.5000   10.0000

a2 = filter([0.5,0.5],1,a);

b = zeros(2*max(size(a)),1);
b(1:2:end) = a2;
b(2:2:end) = a;