Duped

I've got little question . I've switched the Mnova spell from my inventory into my equipment (while i worn the T0 Spell). I changed them a bit diffrent like normaly . i switched them by double clicking very very fast and holding the spell by 2nd click . there it was ... the t0 spell was gone and i had 2 mnovas ... . could someone explaim me how this worked? btw both spells were real and tradable ... and no its no april fool !!!