No, multiple posts with the same CPT and post name (slug) are not possible. Someone can work hard and ruin the DB by writing into it some illegal values, but that should not be something you think of when writing your code.
Posts in their initial state are not true posts, they are just place holders, therefor while your observation is valid, it is not really relevant for usual usage pattern of wordpress API, unless once again your aim is to break things.
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