If you want to create Control Fields, especially multi-layered Control Fields, you will need more than one key from some portals, but portals will not drop keys if you already have a key for that portal in your inventory.

Drophacking is your solution. Before hacking a portal, you can click on the portal and in the bottom left of your scanner you can see if you already have one or more keys for that portal in your inventory. If you do you can simply click on the portal key icon and it will bring you to the portal key in your inventory. Simply select the "Manage" button and "Drop" the Key. Be sure that you don't "Recycle the Key".

If you have more than one Key keep dropping them until you have no more left. You can then acquire new keys through hacking. After finished hacking, just pick up the keys that you have dropped from the Ingress scanner game map. Be careful that no enemies are around when you do that though, as they may take advantage and collect your keys while you are busy hacking.