4 days ago I caught a Dratini in an alley at midnight. Today I found another Dratini in the same spot at the same time. I'm wondering if this is just dumb luck or if it actually stays there. I'm going to the same spot ,tomorrow, to see if it will be there again.
Is there a guarantee if it will be there? How long will a Pokemon stay in the same location?
You cannot catch the same Pokémon twice. You can only catch another one of the same kind, which happened in your case.
If you catch a Pokémon it is gone for you, since you have it in your bags now. Going back to that place won't spawn the same Pokémon again, unless you're on a different account and within a few minutes. After 4 days it was definitely a different Dratini, especially if you already caught the other one.
There is usually a chance every hour at the same minute that a Pokékom spawns at a given point. But this Pokémon can be of any type. (But some types have higher rates in different locations). So for your Dratini it is possible to spawn every hour at minute XX and so it will also spawn after 24 hours at minute XX. Maybe you are lucky to find two times the same Monster or it's a spot where only Dratinis have a spawn chance.
Theres a good chance. Pokemon stay around the same area so while you cant calculate it up 2 "hey every 24 hrs im running here for a dratini" but where you find a pokemon you like visit there frquently they stay in the same area. Cheers hope that helps