Sorry if it's somehow unrelated but, I thought leaving this bit of info could bring more ideas for you T500RS owners, on how to solve your problem with 2x downshift with the paddles...
On the Logitech DFP, ever since its release in 2004/05, it's common to get this exact same issue, though more often with the sequential-shifter than with the paddles (different components, but same mechanical principle).
What happens is that the "clicking" timings on the button/shifter/paddle are received, or perceived, too instantly, too quick, almost as "spiking" (not sure it makes sense).
The solution for the Logitech DFP is to assign a key to the shifter (or paddle) by setting a new command, in the Logitech Profiler software, in a specific game profile, where the key/button "press-release" timing is set between 0.2 and 0.4 seconds.
I'm not sure that the Thrustmaster equivalent software can do this too but, with the Logitech profiler software this is done in the following steps:
The following example/guide is for the Logitech DFP "stick shifter forward" (for downshifts) but notice that this can also be done for the "stick shifter backward" (for upshifts) and/or for each of the two paddles, if you wish. You just have to reassign a different key for the command, for each of these.
So, here we go:
1 - Open Logitech profiler
2 - Create a game profile (click "Profile", then "new", browse to the specific game executable, and give a name to this new profile).
3 - You'll now see a picture of the wheel with all buttons, paddles and stick-shifter.
This "fix" works for any of its buttons, paddles, stick-shifter, even pedals.
Just to illustrate this for an example in the DFP, we'll use the "stick shifter forward" (generally used for downshift).
3.1 - Now mouse over and LMB click the "stick shifter forward", a little menu pops up.
Now select the option "Select Command", then "New Command".
3.2 - A new window pops up... you'll now have to assign a key and a name for the new command.
We'll use the key "M" just for this example. So, for the name, you can put "M" (without the quote marks), same as the key you'll be selecting. Though you may put whatever name you wish ("downshift", etc, etc).
3.3 - Now click in "options" and put a check on the "record pauses", and click "OK" (this is so you can see how long it takes, in seconds, to press and release the key/button).
3.4 - Now, click "Record", and press then release the desired key in your keyboard (for this example it's "M").
You'll want your "press and release" exercise to be between 0.2 and 0.4 seconds (no more, no less).
After this is done, click "OK".
3.5 - All you have to do now is to re-run the game for which you've set the new Logitech profile, and reassign the downshift command again (in that specific game options).
You'll see that, in game, while reassigning your stick-shifter-forward for the downshift, that it is now recognized as "M".
The end result... no more double-downshifts issues, just smooth and precise downshifts!
(click image to enlarge it)