Doppelgang | Echo Mage | Corridor Monitor

Cards

Doppelgang in hand.
Echo Mage and
Corridor Monitor on the battlefield (
Echo Mage without summoning sickness and is level 2 or higher).

You control a mana-producing permanent that enters the battlefield untapped and that can tap to produce at least ({U} magic symbol) .

Cast
Doppelgang by paying ({6} magic symbol) ({G} magic symbol) ({U} magic symbol) , targeting
Corridor Monitor and a mana-producing permanent.
Holding priority, activate
Echo Mage by paying ({U} magic symbol) ({U} magic symbol) and tapping it, copying
Doppelgang, targeting
Corridor Monitor and a mana-producing permanent.
Resolve the copy of
Doppelgang, creating two copies of
Corridor Monitor and the mana-producing permanent.
When the
Corridor Monitor tokens enter, they trigger, untapping
Echo Mage and any other applicable permanent.
Holding priority, activate the two mana-producing permanent tokens by tapping them, adding at least ({U} magic symbol) ({U} magic symbol) .
Repeat from step 2.

If desired, you may only resolve one trigger in step 4. This would allow the next iteration to target any other permanent instead of
Corridor Monitor. You can then use the second trigger to untap
Echo Mage again and proceed.