Police in Concord are investigating some hate-filled graffiti left on the home of an African family, similar to graffiti left on three other homes last fall. The message discovered Sunday morning, like the ones left on the homes last year, was written in permanent black marker. It calls the residents ‘‘Third World scum.’’ Messages left on the homes last year referred to the occupants as ‘‘subhumans’’ and said they are not welcome. “The sentiment is just about the same . . . now we have something to compare,’’ Lieutenant Timothy O’Malley said, referring to the handwriting. No one has been arrested in connection with the cases last year, which also were investigated by the New Hampshire attorney general’s office. The families targeted also are from Africa. They did not know each other well.