The form supercede is commonly considered a misspelling of supersede, since it results from confusion between Latin cedere (“give up, yield”) and sedere (“to sit”). The original Latin word was supersedere (“to sit above”), but the ‘c’ spelling began to be used in Middle French, appeared in English as early as the 1400s, and is still sometimes found. The fact that 'supersede' is the only English word ending in sede, while several end in cede, also encourages confusion.
Most dictionaries do not include this spelling; a few list it as a variant, sometimes identified as a misspelling. A search of general dictionaries at Onelook All Dictionaries finds 4 instances of "supercede" excluding this one (with one flagged as misspelling), and 24 of "supersede".