I keep myself up to date by checking the 'recently added' list. This list conveniently shows a yellow 'v' for objects I already own but now have an update available, or a shopping cart for objects that already are on my wishlist.
But there is no easy way of knowing if I already have an alternative.
Sometimes there are 2 mods available for the same airport, or one mod including some hand crafted buildings in a city, while the other has photogrammetry for the entire city.
Can we have a tag showing that we already own the alternative mod? (or have it on our wishlist)
Alternative mods could also be added to the bottom of the mod page.
You could let the community label mods as alternative mods to reduce the workload for moderators. (or let them flag it, so mods only have to check it)