In the old weightlifting rules (I think this changed in 2018?), if two lifters were tied with the same lifts, the medal would go to the lighter lifter. Now the lifter who makes the lift first takes precedence and lifters who follow will need to exceed their lifts to outdo them rather than match their lifts at a lower body weight.
Weightlifting
By Weightlifting, you mean Olympic Lifting.
Same Lift
I am not familiar with the Olympic Lifting Rules. However, I suspect that the same is true as in Powerlifting; since Powerlifter pretty much used Olympic Lifting as it's templet.
I did a cursory reading of the Olympic Rule book but didn't find the rule on this.
With that said, the first lifter in Powerlifting to set the record in a individual lift (Squat, Bench Press, or Deadlift), owns the record, regardless of if they are heavier. Since there a so many Powerlifting Organization, it hard to keep up with the rules in all of them.
That the was rule of the first Powerlifting Federation. The first to set the record, owned it. The weight of the lifter did not come into play.
Lifting Totals Match: Lighter Man/Woman Wins
The rule of the lighter man/woman winning over the heavier lifter is the rule.
If both lifter end up with the same total, the lighter lifter wins.
I think weighing-in after the event could mess up the flow of competition, especially if a competitor is successful in all of their attempts only to turn out to be overweight at the end of the competition and is disqualified (other lifters may have been pushing themselves unnecessarily hard to match/outdo an opponent who should not have even been eligible to compete in the first place).
Weighing After Lifting
This is an interesting idea with a lot of issues to resolve.
It would be hard to declare which weight class you would be lifting in. Thus, hard to know who you were lifting against.
Also, a lighter, stronger lifter could move up and displace heavier, weaker lifters in the weight.
Example
A buddy of mine in the 165 lb class was much stronger than I and the rest of the 181 weight class.
The 165 lb class was brutal.
My buddy weighed right in at 165 lbs. It would not have taken much eating or drinking to get him over the 165 lb weight and then into the 181 lb class.
He would have placed first in the 181 lb weight class just above 165 lbs rather than second in the 165 lb class.
None of we in the 181 lb weight class would have seen it coming or know until the end.
Disqualification Due to Being Overweight
I question if this would happen. If it did it would create some health issues from lifters having to watch what the ate or drank during the competition. Some might need to continue to weigh on the meet scales prior to eating or drinking something...crazy crap like that.
In Meets In that require a 2 Hour Weight In, if your weight is more than the Weight Class that you listed on your Registration Form, you are then place in the higher weight class. You are not disqualified. Most likely that would be what would occur with Post Meet Weigh In, as well.
The USAPL follows weightlifting-like procedures, including the 2-hour weigh-in.
2 Hour Weight In Rule
This was the rule with the first Powerlifting Federations. It was/is a good rule.
I am not sure how we migrated to weighing in around 24 hour ahead of time.
To some extent, early Weigh In had to do with large meets.
Some meets would have 100 lifter. It took forever to get everyone weighted in.
That is one of the reason that some meet allow you to weight in the 24 hour before the meet as well as the night before.