With the whole Zone 20 craze, I figured this should be applicable in other areas too. And having tested this with a friend out for a bit, can confirm they work reliably well for the Chespin spawns. You're able to bring up the total spawns from 3 to 4(Hey that's a 25% increase), which can alleviate the pain just a little.
Anyways, it's the same principle as the Zone 20 method, the screenshots should give you an idea of where each important save and reload spot is.
Here's a quick summary on the steps I followed:
1. Go inside Zone 17 while being undetected by the mound spawns and KO the Pyroar pack.
2. Come back and try to aggro as many Diggersby as possible, I got mine to work with 2 on the first try but if you do lesser you will just need to note down where the original mound spawn was. If they lose aggro then save and reload and their spawn point is moved to the current spot they're at.
3. Bring the Diggersby as close to the Gate in Picture #2, and save and reload outside the gate.
4. Move about 60-65m away from there, refer to Picture 3 for this, save and reload there.
5. Run back through the same gate and check the mounds, while also making sure to not run too far to despawn the Diggersby at the gate, can refer to the pin marked on them and not exceeding 70m. If you see 3 new mounds then you're done, save and reload. If not I would suggest just redoing the whole process from Step 2. 
6. You should be able to Fly or Bench Reset and that should move the duplicated spawn back near the original spawns, and 4 seems to be the cap so don't need to try going for more.

Hope this helps someone out! 

Overloading Chespin Spawns in Zone 17