From an implementation perspective, the only change to make in order to implement DDQN is in the training phase. You just need to replace the following lines of code in the DDQN implementation itself:
mb_trg_qv = sess.run(target_qv, feed_dict={obs_ph:mb_obs2})
y_r = q_target_values(mb_rew, mb_done, mb_trg_qv, discount)
Replace this with the following code:
mb_onl_qv, mb_trg_qv = sess.run([online_qv,target_qv], feed_dict={obs_ph:mb_obs2})
y_r = double_q_target_values(mb_rew, mb_done, mb_trg_qv, mb_onl_qv, discount)
Here, double_q_target_values is a function that computes (5.7) for each transition of the minibatch.