10% of the time is nothing. And a reasonable amount of time to mess up an input.
This has everything to do with being a newbie. He says he sometimes gets an additional input before or after getting the double button press. This indicates that the system is reading one of the buttons a little early or a little late. Which means he is pianoing the inputs, or releasing one button slower than the other.
I too find the inputs easy as I have also played a couple games. But I have a mate who can't get it down no matter what. People are different, let it be. You may have had some issues with your stick. I can't say if that is legit or not. As I haven't had any issues with the sticks you have mentioned. Or any others I use at the gaming meet ups. But it doesn't mean everybody is having the same problem. And I just feel it is not in the OP's best interest to be told that it's his stick. As that could be a cop out to not practice anymore and blame it on the hardware.
In you other post you mentioned you tried the UFB and had the same problem. That's an indicator that it isn't the stick. The "stick" is the PC board that is inside of it, removing it and using a UFB means it is now a completely different product. The buttons and the joystick used are the same in the old boxes as they are in the new boxes. I use a UFB in my own stick and have had zero problems.
Same people may be having issues. Some may just need to practice. And since we don't know what the person on the other side of the PC screen is experiencing, the best answers we can give are to practice. And advise on what could be causing the problem based on technique. At the end of the day it wont hurt the person asking. And if it turns out they do have a faulty stick, they still got all the necessary practice in that they should have been doing anyway.
I second Darkstalkers. If Nintendo could lock that down as their exclusive fighter it will kill. MS has KI, Sony has that other game, Nintendo could shine. And hopefully it will make its way to Evo and boom, cult following wanting in and Nintendo sells a bunch of Switches.
No tricks man. You just have to do it. The fact that you can recognize you are slipping indicates that you understand the issue. Now you just need to not let it slip. It will all come down to you and nothing else. But here are some ideas that could help.
1) Take a break every hour and come back to it.
2) Set an alarm on your phone. Have it snooze for 15 minutes. So every time it goes off you remember you are meant to be focusing on something, then hit snooze and it will remind you in 15 minutes again. After 4 reminders it will be an hour. Take your break and walk around. Or watch some top level matches to get hype an motivated again.
3) Every time you realize you have slipped up, punch yourself in the nuts. If you are a female. Put two nuts in your lap and punch them. Should work either way.
When it happens you will get a lot of data about yourself. If you feel you start sucking you will be able to pick out what you are doing wrong and you can call yourself on it. Then before you start you next game, say to yourself, "don't do situation A because you don't want outcome B".
And lastly, when you are having this issue, remind yourself you are a piece of shit and if you don't sort your act out you can kiss victory good bye. Decide on how you want to start the round and start that way, say it out loud and not just in your head. Then before round 2 or 3 starts say it out aloud again how you want to start those rounds. It will help you stay mentally aware that there is a specific way to play for victory and that you have a game plan. And with that game plan you will get what you want.