blizzard. Star craft 2 of the top of my head. I got discount on Nova addon, becasue I already had expansions. if i think so more, I can find more examples of discount for an addon, if you have prior parts of it. oh there we go, slightly different situation but I paid $25 for Fallout 4 season pass instead of $40, they gave people a chance to early adopt at a lower price, due to changes in scope of expansions. I also paid $25 instead of $40 for Uncharted 4 stand alone DLC. so... there's that.
its not unheard of.
getting discount on buying Forsaken if you already have prior expansion - before that roll in happens, would actualy be a decent move by Bungie. but instead... they are giving crappy cosmetics if you buy it now at full price. meh. its not at all encouraging me to buy it.