Thicken stew/sauce twice with flour

Question

When making a stew, I forgot to add flour before the liquid. To thicken the stew, I added some slurry (flour + cold water). However, it was not enough. I would add more slurry, but I found this website (2012) that says

If you've prepared a dish that started out with a roux and you feel it needs a slurry at the end for additional thickening: use cornstarch or arrowroot to prepare the slurry. Why? Because a chemical reaction occurs that will prevent the same starch used twice to thicken it any further.

I could find not any other sources supporting or opposing this claim, so I'll ask here:

Can I thicken a sauce/stew twice with flour (roux+slurry or slurry+slurry)?

Answer 1

A reaction that prevents starch from the same plant thickening if you add it later instead of all at the same time seems very unlikely. And anyway, a roux could be made with non-wheat flour even though it's not traditional - what do they think would happen then? Or if you make a roux on another pan and add that. I can confirm that works.

There is a good reason not to add a slurry late in cooking - the raw flour flavour it imparts. That's why I'd use cornflour (corn starch) for late thickening.

Answer 2

If you want to thicken a stew at the end of the cooking process, I've found that the best option is to use a buerre manie. Mix together equal parts flour and room-temperature (not melted) butter, and whisk that into your stew.