Amplify’d from martinfowler.com

Identity Map

Ensures that each object gets loaded only once by keeping
every loaded object in a map. Looks up objects using the map when
referring to them.

An old proverb says that a man with two watches never knows
what time it is. If two watches are confusing, you can get in an even
bigger mess with loading objects from a database. If you aren’t
careful you can load the data from the same database record into two
different objects. Then, when you update them both you’ll have an
interesting time writing the changes out to the database

An Identity Map keeps a record of all objects that have been
read from the database in a single business transaction. Whenever you
want an object, you check the Identity Map first to see if you already
have it.

Read more at martinfowler.com