Aside from the obvious loopholes (how do you define two laws as being the same -- and what stops your opponent from intentionally sabotaging a law they know you'd want to pass?) there are some other practical issues.

If it applied to all laws, you could pretty easily imagine some negative consequences (attempts for marriage equality or suffrage could have been delayed by decades if the opposition tried to introduce laws in a way that would be guaranteed to not pass). And what if the circumstances change within that decade so that an old law proposal now makes much more sense.

I do think that the law-making process should have quite a few more safeguards. The way that after-the-fact amendments work (in America or Australia for instance) is quite scary -- though Australia does have double dissolution which intends to prevent deadlocks and the Senate changing laws without authority from the House of Representatives.

This would mean you get into endless discussion about whether a new proposal is covering the same topic or not. Politicians try to game every system if it helps their cause. And then there are the corner cases where the underlying situation changes enough to warrant action even though it fell through one or two years prior.

We are seeing how legislation lags behind AI (self-driving, facial recognition etc.), we seriously don't want to make that even worse.

The problem is more that the lawmakers need something to do... EU is producing a torrent of regulations, which is a bit absurd since it's layered on top of a substantial amount of already existing legislation in the member states.

It's not so much about "harmonizing the markets" - it's just full speed ahead, high and low, idealistic, corporatistic, amateurish or razor sharp, but all the time relentless building the superstate.