Duplicate existing data field of a content type

1. Edit your content type: Go to Structure » Content types . On the MANAGE FIELDS tab, Add new field. Use the same FIELD TYPE as in the field you want to duplicate. Save
2. Still in content types: Go to the MANAGE DISPLAY tab, and verify that the new field is visible (it is not displayed under Hidden)
3. After the stages above, new database tables will be added: field_data_field_<new field name> and field_revision_field_<new field name>

Put site in maintenance mode and copy old field data to new field's tables:

INSERT INTO  field_data_field_<new field> SELECT * FROM field_data_field_<old field name>;
INSERT INTO  field_revision_field_<new field> SELECT * FROM field_revision_field_<old field name>;

I added org_body field, and copied body field contents to its table:

INSERT INTO  field_data_field_org_body SELECT * FROM field_data_body;
INSERT INTO  field_revision_field_org_body SELECT * FROM field_revision_body;

*The name of the new and old fields can be found in the MANAGE FIELDS tab, on the MACHINE NAME column
Take site out of maintenance mode.