Is this number a repdigit?
Brachylog, 1 byte

=


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This acts on integers.

From src/predicates.pl#L1151:

brachylog_equal('integer':0, 'integer':0, 'integer':0).
brachylog_equal('integer':0, 'integer':I, 'integer':I) :-
    H #\= 0,
    integer_value('integer':_:[H|T], I),
    brachylog_equal('integer':0, [H|T], [H|T]).