I would have thought this was fine, provided all Minimum Connection Time criteria are met, i.e. LON-MAN-LON the first time works, and LON-MAN-LON the second time works, AND you leave a MCT gap between the arrival at the end of trip 1 and the departure for trip 2. Even if you don't do the latter, it's your own lookout (separate PNRs) if you miss the start of the second trip anyway. So it seems like a perfectly 'valid' (if a bit crazy ) thing to do.

I suspect this may be some kind of 'clever' revenue protection system gone wrong? For example, John Q. Businessman has meetings in MAN but doesn't know quite what time he'll make LHR after meetings in London. He checks prices and sees it's cheaper to buy 2 el cheapo returns and throw one away than one flexible one in the first place. BA's system cries foul (and in this case is wrong anyway as IIRC to qualify for the 50K they were all top-whack, not-cheapo classes). But if you insist you intend to take both (and your reason is both perfectly valid and plausible!) then so be it and thankfully they can 'protect' the booking somehow.

I shouldn't worry too much if they've agreed to 'protect' the booking...just keep an eye on MMB...

Phil