I agree with Maciej, this should be done with SQL CLR. Personally I have made something similar for my company, except that it calls Navteq Web Services and stores...
Personnaly I would store in a datetime when the respondent was ignored (you must store date AND time), then a simple stored procedure ran every minute by a SQL job...
As SQL tells you, you can't use DISTINCT on a geography column. What do you store exactly in your geo columns ? If they are simple points, you can use...