Jesus did not say he would build his church on Simon Peter. Jesus called Simon "Petros" (a movable stone) and then followed that the church will be built on "Petras" (an immovable rock). Jesus is the Petras that the church is built on.
Jesus spoke Aramaic, not Greek, and said 'Kepha' both times.
Also realize Jesus is God and therefore omnipotent. He would have known at the time at if he worded anything ambiguously, people would do the wrong thing for 2000 years. I think he wouldn't have been ambiguous.
Odd, we don't call him Simon Kepha for some reason. Maybe because Greek was used for the wordplay. So while Jesus spoke in Aramaic he used Greek at this point. Also, since scripture is inspired by God, He had Matthew write Petros and Petras in the Greek text (Matthew wrote his gospel in Greek).
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