MigrateSQLMap with multiple same-name source keys?
I'm not aware of a solution without altering the source database -- although there are in my opinion clean ways of altering the source DB; and it's not a ton of code:

In either your migration module hook_install() function or a pre/post Import hook call (https://drupal.org/node/1343112) simply using a custom db_query() such as:

db_query("CREATE TABLE foo LIKE other_table; INSERT INTO foo FROM (intersection query); ALTER TABLE foo (change id to type_id);");


You can drop/recreate the table as apart of a pre/post import call.

You just have to get the SQL right and it's all of 3 SQL statements, and 1 or 2 Drupal hook functions to make it all repeatable using Migrations.