This is impossible to fix completely. Steam always creates some new IDs for a game and it's hard to have a system that understands how to "combine" them.
you cant manually set like:
ID1
Id2
ID3
they all are Rocket League.
and make a topic for sellers to post the same games with differents ids, if posted you resolve, so you no need to check manually yourself.
We do not have the manpower to manually do this work. Some games like CS:GO have like 40 ID variant, Not to mention the sheer amount of games there are on Steam.
We have an auto system to combine them, but that does not always work. We will try to improve on that. But do not expect this problem to be completely fixed. As long as a new ID pop up you may see double or more entries in the market.