Andrés Manuel del Río
Mineralogist working in Mexico who in 1801 became the first discoverer of the element vanadium, in the Royal Society of Chemistry's own phrase: "Vanadium was discovered twice. The first time was in 1801 by Andrés Manuel del Rio." Paris chemists wrongly dismissed his find as chromium, and the element was rediscovered and named in Sweden three decades later.
Why this wins its question: A textbook case of a Spanish-world scientific first lost to a misjudgment in Paris — stated here not as grievance but as the Royal Society of Chemistry's own recorded history, quoted and linked.
Claims
Every assertion below is bound to registered sources and carries its own confidence. Weight them; do not treat the page as uniformly authoritative.
Working in Mexico in 1801, Andrés Manuel del Río found a new element in a lead mineral and sent a sample to Paris, where French chemists concluded — wrongly — that it was a chromium mineral, and the discovery was set aside.
The element was rediscovered by the Swedish chemist Sefström, who separated it from cast iron, showed it was new, and named it after Vanadis, the Norse goddess — which is why the first discoverer's name appears nowhere in the element.
The Royal Society of Chemistry's element history explicitly records the double discovery and credits del Río with the first.