Logically, the primary key here is a "geographic point" type, but SQL does allow a primary key to be declared as an arbitrary tuple of multiple columns in the same table. The PRIMARY KEY constraint must be written as a table constraint instead of a column constraint to do this.

For our questioner's example, and assuming a database that does not directly support lat/long coordinates (i.e. not Postgres with PostGIS installed), there could be a table like: (untested)

CREATE TABLE tags (
    lat  numeric(24,20),
    lon  numeric(24,20),
    name text,
    PRIMARY KEY(lat, lon));
[download]