The answer is SUPERSEDE.
Whoever is writing the word goes by what they are hearing, and most words ending with that sound are spelt …cede. But the etymology explains differently. The word descends from the latin roots super above and sedere to sit with the meaning sit on top of.
Although the word is correctly spelt supersede, some dictionarys include the supercede spelling. It is such a common misspelling it is becoming accepted as an alternative. Bad news for us pedants.
Citations for supersede
T. F. HOAD. “supersede.” The Concise Oxford Dictionary of English Etymology. 1996. Encyclopedia.com. 9 Oct. 2009.