How to set a column value based upon duplicate records

Suppose I have the following data in a table named "Teams"
If I have a table of football teams.
I want to update the table and set the "Flag" column to 1, if there are any duplicate football teams in the table.
The TeamId will determine if a row is a duplicate, since the TeamId would occur more than once. Here is an example of the table before the update.

TeamId   TeamName                   Flag
BAL          Baltimore Ravens
BAL          Baltimore Ravens
BAL          Baltimore Ravens
PAT          NE Patriots
SF9           SF 49ers
SF9           SF 49ers
SF9           SF 49ers
SDC          San Diego Chargers

Here is how the table would look after the update. All duplicate  rows have their Flag set to a value of "1"


TeamId   TeamName                   Flag
BAL          Baltimore Ravens           1
BAL          Baltimore Ravens           1
BAL          Baltimore Ravens           1
PAT          NE Patriots  
SF9           SF 49ers                           1
SF9           SF 49ers                           1
SF9           SF 49ers                           1
SDC          San Diego Chargers




INSERT INTO Team (ID,FirstName,LastName) VALUES ('BAL', 'Ravens',null)
INSERT INTO Team (ID,FirstName,LastName) VALUES ('BAL', 'Ravens',null)
INSERT INTO Team (ID,FirstName,LastName) VALUES ('BAL', 'Ravens',null)

INSERT INTO Team (ID,FirstName,LastName) VALUES ('PAT', 'Patriots',null)

INSERT INTO Team (ID,FirstName,LastName) VALUES ('SF9', '9ers',null)
INSERT INTO Team (ID,FirstName,LastName) VALUES ('SF9', '9ers',null)
INSERT INTO Team (ID,FirstName,LastName) VALUES ('SF9', '9ers',null)
INSERT INTO Team (ID,FirstName,LastName) VALUES ('SF9', '9ers',null)