Two identical uses of \tag will cause identical element id's

Since the \tag{xyz} macro produces a DOM element with id="mjx-eqn-xyz", two identical uses of \tag{} will cause two DOM elements to have identical id's, which is technically an error, since element id's are supposed to be unique.

Since MathJax uses the element id's to locate the elements associated with its internal MathML structure, that can cause confusion if the id's are not unique. See this post on meta.math.stackexchange.com for an instance where this occurred in the wild.