check value in datatable

I have string value in the column of datatable.
I would like to assign seq no to different value.
if they have the same value, I would like to assign the same seq no.
if there isn't has the value in the column, then assign a new seq no.
how to do this?
for example:
district no.    district
1                  hong kong
2                  germary
3                  canada
1                  hong kong 
...