Here is the law I was referring to (it is interesting that
Singapore has such a law despite its stance on dual nationality -
it actually states "whether or not the person is also th national
of a country other than Singapore")
Person entering or leaving Singapore to produce
passport, etc.
5A. —(1) Subject to subsection (3), every
person, whether a citizen of Singapore or a non-citizen, who is
arriving in Singapore (by air, sea or land) from a place outside
Singapore, or is leaving Singapore (by air, sea or land) to a place
outside Singapore, shall present to an immigration officer at the
authorised airport, authorised landing place, authorised train
checkpoint, authorised point of entry, authorised departing place
or authorised point of departure, as the case may be —
(a) if the person is a citizen of Singapore (whether or not the
person is also the national of a country other than Singapore) —
the person’s Singapore passport that is valid, and any other
prescribed evidence of the person’s identity and Singapore
citizenship; or