Resolve Bug IOT-273 (rename "OC_STACK_REPEATED_REQUEST")

Resolve Bug IOT-273 (rename "OC_STACK_REPEATED_REQUEST")

 - OCStackResult "OC_STACK_REPEATED_REQUEST" is renamed to
   "OC_STACK_DUPLICATE_REQUEST"
 - In StringConstants.h, "REPEATE_REQUEST" is rename to
   "DUPLICATE_REQUEST" (Message string also changed)

Change-Id: Ie3bca4642441a59c3c66a98513d0f15d2e923096
Signed-off-by: ywsung <yw.sung@samsung.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/177
Tested-by: jenkins-iotivity <jenkins-iotivity@opendaylight.org>
Reviewed-by: Sashi Penta <sashi.kumar.penta@intel.com>
Reviewed-by: Joseph Morrow <joseph.l.morrow@intel.com>
Reviewed-by: Sudarshan Prasad <sudarshan.prasad@intel.com>