These not really are snippets but, these are useful things which prevent some annoying bugs, or dupes within vanilla GUI's

• Anti "Move all items from container to player's cursor" dupe
  Requires skript-mirror:

Code (Text):

1. import:
2.   org.bukkit.event.inventory.InventoryType
3.   org.bukkit.event.inventory.InventoryAction
4.  
5. on inventory click:
6.   if event.getWhoClicked().getOpenInventory().getTopInventory().getType() is InventoryType.CHEST: #You can change type to your's gui Type.
7.     if event.getWhoClicked().getOpenInventory().getTopInventory().getTitle() is "Name of chest":
8.       if event.getAction() is InventoryAction.COLLECT_TO_CURSOR:
9.         if "%event.getWhoClicked().getOpenInventory().getTopInventory().getContents()%" contains         "%event.getCursor().asOne()%":
10.           cancel event

• Anti "lock item in slot that contains air" whenever inventory click event is cancelled
  Requires skript-mirror:

Code (Text):

1. import:
2.   org.bukkit.event.inventory.InventoryDragEvent
3.  
4. on InventoryDragEvent:
5.   if event-inventory's name is "Title name of inventory":
6.     cancel event