What happens if I make the same assertion twice (especially with tell and ask)?

This really depends on the  assertion. You should look into the specs  of the operation individually.

For  example, if the assetion is the creation of a frame of a defined name and the behavior :frame-name-required is true, this will generate an error.  If you add a slot value that already exists, it will generate an error if the :collection-type of that slot is :set.

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