Thought I'd better check back in here, as having quite a relapse recently. Mercury box set now forming major part of my listening, as also the EMI Karajan orchestral box. Re the latter I can provide some comfort for those who have not yet succumbed to that. There are multiple copies of different pieces, and some of them are old and some are in mono. Some of the performances are - IMO - a bit dull, though interesting as a historical record. I'm thinking of the Handel Water Music in particular (disc 86) - they don't play it that way these days! I do wonder about the almost as large EMI set containing vocal/opera music, whether that is good, or is also somewhat flawed by repeat performances. If I don't buy it before Christmas do I get a gold star?
I bought that one when it came out. It has two Meistersingers, two Trovatores, two (Beethoven) Missa Solemnises and two Brahms Requiems, and I think no other duplications, so that's about 8 of the 71 discs duplicated. Similar thoughts to yours - plenty going back to the 40s and 50s in mono, though all of reasonable sound quality. Generally it's the newer recordings I find less interesting though.