I think I found a small bug:
I named one of my GUIs "gPlayerDeck" and then tried to name one of my characters "cPlayerDeck". I made sure the first letters are different, so technically they are different names. But I still got an error:
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Adventure Game Studio
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This script name is already used by another item.
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OK
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I can get around it by naming my character cPlayerDeckc, but that's annoying. Is it a bug, or did I do something wrong?
It's something with the internally workings of AGS. If you were just to type Pla the autocomplete would come up with PLAYERDECK next to a gray box. It is an automatically created define, and I think it does it for all Guis, Characters, and Views.
To solve, rename your Gui or Character.
~Trent
It's to maintain backwards compatibility with previous versions of AGS, where characters, GUIs and Views were accessible with a capitalised name, as Trent has indicated.