Specificity - CSS: Cascading Style Sheets | MDN
developer.mozilla.org
Specificity is the algorithm used by browsers to determine the CSS declaration that is the most relevant to an element, which in turn, determines the property value to apply to the element. The specificity algorithm calculates the weight of a CSS selector to determine which rule from competing CSS declarations gets applied to an element.

Adding the same id multiple times increases the specificity of a selector and thus overrides other selectors.

e.g.

#myId span {color: red; }
#myId#myId#myId span { color: blue; }

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