core: Don't use bash for duplicate script check

Use /bin/sh instead of /bin/bash in the script that checks for
duplicates. This helps building in environments where bash is not
available.

Change-Id: Iaaf0d7c448e4b21e1174f41ad5efa9a87a399d98
Signed-off-by: Juan Hernandez <juan.hernandez@redhat.com>