Yes, but as the report says it's a reconnaissance helicopter and optimized for target designation. Whereas the Mi-28N is far more of a battle attack helicopter. Sure they overlap, so best option is to have both for different missions and need be perform the same missions. So it's understandable that this split buy approach is taken instead of picking one. Also I think politics are at play, with government wanting to support both companies, both products. The Ka-52 can do well on the export market, but only if the Russian Army supports it as well. The Mi-28N not so much, but is far more in line with what the Russian Army wants. Also, the Ka-52 wasn't ready for production, where the Mi-28N was, we're talking about at least a four year gap I think? And for especially special operations support, they needed a night-capable attack helicopter fast. Also I think (not sure), that the Mi-28N costs far less than the Ka-52. So opting the best overall performer was maybe more expensive than a mixed fleet despite the total costs. If they looked that far into it... *skeptical*, seems mostly a requirements decision anyway, not financial.
They use the same engine, so I'd say 150% the maintenance. Take in other systems which are the same and it may be even less of a concern.
Double the training maybe in the long term, but not really double either I think, as the specialized roles on the Ka-52 would mean additional training anyway. So it's only the conversion, based on the number of new helicopters and number of pilots flying Mi-24s nowadays, it's not that big a deal either I think.
Spare parts inventories, see point about maintenance, but yeah sure, more costs, but keeping two products in the export catalogue might justify that. Double parts production, well not really, a bit more maybe, lower numbers so less economy of scale, however keeping multiple manufacturers order books healthy might again justify it. On the other hand, if these companies manage(d) to introduce production/inventory management of roughly the 80s and 90s, it won't be that big a deal to deliver spares when needed. Remember, the Russian military does not fight much wars overseas, requiring extroardinary logistical efforts, where due to shipping times to the forces they need to keep bigger inventories of spare parts to keep availability numbers as high as in NATO. This is Russia, spreading risk across two types and two producers is more important than keeping up high maintenance levels, as you can't rely on one producer only, plus because of conditions and uncertainties you certainly can't rely on one type either. Finally, defense budget is rising, not shrinking, so getting the best is more important than selecting on costs in order to reduce spending as the case with many other militaries.
Everything said above also applies to fighters, which is why you'll keep on seeing a mixed fleet for a long time. The fifth gen PAK-FA should finally make an end to Russia's lo-high fighter mix, but it will keep on having different aircraft for more specialized roles, and this Ka-52 is more of an example of that than it appears to be because in principle it perform the same basic roles. Then it would probably requiring another Mi-28 version, which would also come with additional costs, and frankly is a dated airframe already.