Does dirt under the fingernails or dust on the feet invalidate wudu', and what is the stronger opinion regarding a small amount of dirt versus a large amount, and is it permissible to wash the feet twice during wudu'?

The questioner, who appears to be afflicted with waswas (obsessive whispering), is advised to disregard it. Regarding dirt under the fingernails, scholars have differed on this issue, with many holding that it is excused as long as it does not exceed what is customary, citing various sayings and rulings that support this view. Similarly, dust and dirt on the foot or other parts of the body do not affect ritual purity unless they form a barrier that prevents water from reaching the skin. Washing sandals and feet for dust is an reprehensible form of tanattu' (excessive scrupulosity), and the only obligation is to wash them for wudu' (ablution) itself. The questioner is advised once again to stay away from waswas.