Duplicating items:
1) Open the Creative Mode Inventory
2) Select item and place them to "Survival Inventory" section, on hotbar
3) Throw it (hovering on them and pressing Q with opened inventory)
4) Pick up item (without closing inventory)
// 5) Be happy because your diamonds are be duplicated xD

Disappearing items:
1) Open the Creative Mode Inventory
2) Select item and place them on hotbar (expect "Survival Inventory" section)
3) Throw item (hovering on them and pressing Q with opened inventory)
// 4) Try find them

In annex I am sending images for rectification.

(Sorry for my bad English)