I agree that passing the measurement when entering the line and then not the second time is a lot for a kid in the spectrum to process. The sudden ride denial will feel so unexpected. It a lot of disappointment for any kid. Why not just teach the first CM how to measure accurately and eliminate the second measure. You are either 44" or you aren't. It shouldn't be a matter of opinion. No other measurements in life are opinion based.
The issue isn't that the first CM doesn't know how to measure.
It is, as wilkeliza mentioned, because people can and do try to sneak a smaller child in. I have seen someone who appeared to be trying to do it myself several times. Either one parent tries to distract the CM while the other parent enters the line with the child, or they kind of try to 'sandwich' the child between other members of their party while walking in.
Maybe people don't try that at much at other parks, but it happens at Disney fairly often. That is why they have the second check - because people are not honest.
And that is also why they don't do height wristbands. They did do it for a while - they had even gotten high tech laser measuring devices and built all new measuring areas. But, people cheating put an end yo that pretty quickly - from what I heard from CMs, some people came prepared with scissors and tape. They got child one measured then transferred the band to child 2 who was too short. Then child 1 got measured again = 2 kids with wristbands that said they were the right height.
I think there is more incentive to lie at Disney World - for many people, it is a once in a lifetime or every couple of years thing, so people feel like 'this trip' is their one chance for the child to ride. From what I know, the other parks mentioned get a lot more repeat visitors, so if a child is too short, there is likely a visit coming up soon.
Whatever reasons, Disney is not likely to change.
The 2nd check is the real official check. They do it there in case someone tries to sneak a younger one on. I know I've seen kids who are inches short get through at the front for whatever reason and the 2nd check catch them.
The important thing is to let the little ones who are on the cusp know they need to hit that bar so stand up straight. Some little ones who get pulled for the at ride check think they have done something bad or get upset so slouch and that is why they get told no. Just remind then to stand up tall and they should be fine.
This question has come up before and what was advised was preparing ahead of time.
1) let the child know they will be measured twice and if they are not tall enought
either time they will not be able to ride.
2) most kids are afraid of bumping their head and many will even duck to avoid hitting - it's natural instinct - how often do we go around trying to bump our heads?
That is the main reason kids fail the height measurement.
3) Practice! Practice! Practice!
When this has come up before several times, we suggested actually making a measuring device. It's a stick with a cross bar at the top, kind of like an upside down L.
Practice having your child stand under it and hitting the bar with his head. Make a big deal over it when he hits it. That will teach him what to expect and what he needs to do if he wants to ride.
Everyone who was worried they would have meltdowns over the second measurement came back and said it worked great.
4) Ride early in the day if you can. As the day goes on, people get tired and hot and start to slouch. So, a child who measured OK earlier in the day might be too short later.
5) the final thing would be to skip the attractions you think your child might be borderline for. If they are not going on, there is no chance of getting measured.