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Either spelling is correct...

Definition of SUPERCEDE
variant of supersede
Usage Discussion of SUPERCEDE
Supercede has occurred as a spelling variant of supersede since the 17th century, and it is common in current published writing. It continues, however, to be widely regarded as an error.

http://www.merriam-webster.com/dictionary/supercede

And, according to another source:

Supercede is a fancy-sounding word for a simple concept: if someone cuts you in line, he supercedes you. To supercede is to take something or someone's place.

To supercede is to take someone else's position. A basketball team that moves into first places supercedes the team that was in first place before. Also, non-physical things can supercede each other — one law could supercede another. Supercede is considered a variant spelling of supersede, the more common way to spell this verb.

http://www.vocabulary.com/dictionary/supercede

Personally, I've always used the supercede spelling...
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