here is my question: If someone from the US buys a foreign
general aviation aircraft that already has a foreign-format identification
already painted on it, then brings it to the US, does that person have to
have it licensed by the US FAA, and have a US-format N-number painted on
the
fuselage?

No. The aircraft can be registered anywhere, as long as you maintain it
according to the country of registration's laws. There may be time limits on
the importation of foreign registered aircraft in the US. I don't know.

Does the foreign identification number have to be removed?

Only if you register it in the US.

Can
both numbers be displayed?

No. The plane must only display the registration numbers of the country it
is registered in, and it may only be registered in one country.

JMO
Harvey