Parking and dupe content

Im new to parking domains so bare with me.

I have a domain that has 1000's of pages indexed - want other domains same name to rank in its own country.

For example mydomain.com is pointing to mydomain.us ( same site ).

Will this not be dupe content in the eyes of google and some how get a slap for this.

Plan to buy domain name for many countries so need some feed back on the best way to do this.

Been reading a bit on this and found the best way round it is to redirect, but I would have thought this would give the other domains no point as all are redirecting to mydomain.com.

Any thoughts - If I am right (above) - why have parked domains.