Its really annoying and offputting that DLC is still shown as available to buy when I have already purchased it.
It clutters up the menus and seems to do nothing but make an underhanded, sneaky attempt to get people to purchase dlc again by accident. They should be removed once the dlc has been purchased so the menu is not cluttered. Its a great game, you dont need to emply cheap tactics to get extra money, just give us DLC each month and we will be happy to pay if it adds value

There should be a single menu on start area that lists all the DLC with iconds beside ones already purchased (including a tick beside each part of a dlc combo pack to show if you already have one of/both the parts already as single dlc downloads)

The fact that there is a warning that DLC can be purchased multiple times (eg on its own and then part of a combo pack) shows that this is a known issue.
It cannot be difficult to scan the users machine and notify that DLC has already been purchased and point them to the other single dlc that makes up the pack.