The question of plagiarisim is as old as pop itself, although George Harrison is the most famous musician to fall foul of the lgeal implications, with appropriate media noise based on his status and reputation.
The case concerned Harrison's 'copy' of the 1963 Chiffons hit single 'He's So Fine'. Contrary to popular myth, the writ was not brought by producer Phil Spector, but by the song's writing team Phil and Mitch Margo, Jay Siegel and Hank Medress, known collectively as The Tokens, and the publishing company Bright Tunes. Concluding in court on 26 February 1981, the case exposed the minefield of plagiarism, and the difficulty in trying to establish copying as a deliberate or subconcious act, and then trying to pin it down as a legal concept. After much legal argument about what actually constitutes a 'song' and a 'melody', not helped by a judge who allegedly was unaware of who George Harrison was (!), George was found guilty, and forced to pay back royalties of $587,000. The ludicrous nature of the case was underlined by Harrison admitting in court that he was indeed copying a hit song, but it was actually 'Oh Happy Day' by The Edwin Hawkins Singers, which reached Number 2 in 1969!
Plagiarism suits continue to be brought to this day - the most recent being Robbie Williams for 'borrowing' aspects of a Woody Guthrie song for his own hit 'Jesus In A Camper Van'. The frequency of writs is often proportional to the fame, and bank balance, of the accused, but there is still no hard and fast legal formula laid down. You go to court and take your chances.