So, buying a thing you already own does not give the option to increase your total donation, to reach some "dlc".
Instead you get exactly what is written on the tin, if you click purchase again, you buy it twice.
I do not agree that this is misleading in any way. The button is labeld purchase and not increase amount or something. There are games that give you a steam key and such.
But I do agree an increase donation somewhere where your "you own this game" is written might be a good idea.
0 Demo Version
10 Base Game
20 Extra features
You cannot buy the base game and then buy the extra features, if the game pricing is like this. Might explain why I saw DLC as separate projects.