Double PD (For both right and left eye) is very important to make your glasses accurate. Its very easy to measure at home. 

1. Take one ruler and mark your nose center zero and stand in the front of mirror

2. Then keep ruler (zero point) from the center of your nose and measure till the center of your right pupil. Make sure your left eye is close. whatever the number falls (Acc to ruler)on the center of your right pupil, Thats your right eye PD

3. Same you need to do with left eye.

4. once you measure both eye pds, please re do it to make sure Both eye pds are correct. You can check picture below.