I had a problem occasionally with double and even triple down-shifting. Caused me to blow my motor a couple times, and spin often. I thought my shifter was wearing out, or perhaps my shaky hands were the problem; until i read a post recently about someone else having the same issue... and found a fix!
Within the documents folder for iRacing is a file called app.ini. Within this file (which you can open with a text editor) is a line that says "debounceSeq_Ms=80". This allows for shifting changes within 80 milliseconds. Don't know about you, but i haven't moved that fast in years... well not intentionally anyway.
HINT: If you're unfamiliar with text editors, generally there is a function within the 'Edit' pulldown that allows you to 'Find' the wording you want to alter. Either type or copy and paste what you want to find in the window, and hit 'Find Next'. Magic!
The original post suggested altering the 80 to be 250 (or 1/4 second). This way, any accidental shifts that occur before a quarter second has elapsed won't be registered. Voila! Up shift and downshift errors remedied.
Just from my own personal experience, before you alter the .ini file, first save it as app BAK.txt. That way you'll always have a clean backup copy to refer to if you need it. After that, If your text editor doesn't show .ini as an option to 'save as', you should be able to alter your 'Save as type:' drop-down window from 'Text Documents (*.txt)' to 'All Files'. That should allow you to now see the original app.ini file along with the app BAK.txt that you just created. Click on 'app' and then Save.
I've altered the app.ini file a few times in the past for other reasons and like to keep it handy, so i keep a 'working' app.txt file on my desktop. That way i always have the original 'clean' app BAK.txt in case something screws up, a current working .txt file that i can refer to, and the app.ini file that iRacing uses and i can easily overwrite when i want to make a change.
See you on the track!