It
is
real. As a matter of fact, they commissioned Michael Richards (yes, Kramer!) to write the sign. It's all right there in the trivia section! Go have a look.
'Remember when' is the lowest form of conversation.

Tony Soprano

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While Trivia mentions this;
The sandwich board that Bruce Willis wore while filming in Harlem was originally blank, rather than text, to ensure no one was offended by the racist message. The "I Hate beep was added with CGI in post-production. Some television broadcasts use an alternate version where the sign reads "I hate everybody", which is sometimes erroneously said to be the original version of the sign used for filming, but this too was added with CGI in post-production.
this behind the scenes photo shows what really was written on sign;
http://images3.cinema.de/imedia/3938/1663938,yclV0JryrnrrvfDuNO_NYRiSMlcozjyT5wpxP95fyX7LUiAqkAXxkJMsmuoJYNQHtGeAMcKK97fbpLbH+rGI6g==.jpg