When baking at home, I often bake cookies batch by batch in order to always use the center rack of the oven for even heat distribution. For easy clean-up and to use less bakeware, it is possible to re-use parchment paper on the same baking sheet. However, it is important to remember that the baking sheet still carries residual heat from the oven. This heat will give the second round of cookie dough a slight head start compared to the first batch.
The trick is to reduce the baking time for the second batch of cookies by 30 seconds to 1 minute. I recommend setting a timer for a minute less than the required baking time. Check on them at this point and if they still need more time then allow for it while watching carefully. Cookies can turn from golden brown to brown brown in a matter of only 30 seconds.