How to detect duplicate facts in the knowledge base?

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5

Quick answer:

You want to check for the fact or assert the fact. The or as used in the sentence means logical or which translates to Prolog as the ; operator.

Here is an example

add_imports(Module,Import) :-
    (
        imports(Module,Import), !
    ;
        assertz(imports(Module,Import))
    ).

This is the first part of the statement that checks if the fact already exist.

imports(Module,Import)

This is the second part of the statement that asserts the fact if the first part of the or fails.

assertz(imports(Module,Import))

Note the use of logical or (;) and the cut (!). The cut is used to avoid leaving behind unnecessary choice points.

The example used is actually from real working code from the post

Basic persistency

which in the example is

add_imports(Module,Import) :-
    (
        imports(Module,Import), !
    ;
        assert_imports(Module,Import)
    ).

but you have to understand that when using library(persistency) that it will create the predicate assert_imports/2, thus why the example code for library(persistency) is different.