It is an addition that I have never understood, Livo, especially when it is to a generic rather than to a specific brand. Why add a as a mixture ("Madras curry powder") more of the same spices that one is planning to add individually, especially when the proportions in the commercial mixture are an unknown ? The only possible benefit could be if the Madras curry powder contains trace quantities of spices that one is not planning to add individually, but in such small quantities that it would be virtually impossible to measure them using domestic equipment.
And then, of course, the vast majority of recipes go on to specify that the "mix powder" is only one of several spices to be added. So the typical recipe calls for (e.g.,) turmeric, plus turmeric as a part of the mix powder, plus turmeric as a part of the Madras curry powder[1] that forms a part of the mix powder.
I continue to remain completely baffled by this arcane practice.
** Phil.
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[1] Khanum Madras hot curry powder ingredients : coriander, turmeric, chillies, mustard, Bengal gram flour, cumin, salt, black pepper, fenugreek, garlic.