When I heard a radio presenter quote a review of Madge's latest ditty as being 'Abba styled', my first thought was: 'Abba styled' my arse, it's patent plagiarism of
Gimme! Gimme! Gimme!, I hope Benny and Bjorn get royalties.
Of course, I immediately got off my Abba-is-sacrosanct point of view once I remembered that copyright and originality are largely 20th-century concepts. Imitation being the sincerest form of flattery has a far, far longer precedent.
Take Handel (or, more properly, Haendel): when he had trouble thinking of a catchy tune halfway through drafting his new opera, he would borrow an aria that he particularly liked from a previous work of his, and simply put new words to it. If he particularly liked an aria that happened to be by
somebody else, he'd put it in his opera and set new words to it.
And then there's JS Bach, who is quite famous for the concerto for four harpsichords (in B minor, unless my memory is failing), which is indeed most skilfully arranged.
Arranged being the key (ahem) point, for it was first a magnificent concerto for four violins by Vivaldi. There's also a wonderful F minor work in German that you'll find, on closer listening, is actually by Pergolesi. Bach may have waited until these composers were dead before nicking their stuff, though I'd have to check on that, but if I remember correctly, Handel had no such qualms.
I concede, however, that when one's catalogue of works runs into the thousands, one might occasionally have run out of a fresh idea or two.