checkseapp: Detect duplicate keys in seapp_contexts entries.

Presently it ignores duplicate keys in seapp_contexts entries, e.g.
if you were to specify:

user=system seinfo=platform user=bluetooth domain=system_app type=system_app_data_file

checkseapp would ignore the duplicate and libselinux would end up using
the last value defined for the key in each line.

Change-Id: I18cadb0c1bf5a907e6fc6513df65aafed91d76fe
Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>