"Where are we going to go tomorrow?"

I would also like to know

The "a" equates here, as it often does, to the English preposition "to" so "a donde / adonde / a dónde / adónde" can translate as "to where" indicating a sense of movement/direction to a location, while "donde / dónde" translates as "where" and just refers to the location.

So basically if we could say "to where" (even if we don't include the "to") then it is "a dónde" or "adónde" in questions and "a donde" or "adonde" in statements. If we can't say "to where" then it is "dónde" in questions and "donde" in statements.

In this DL sentence we can say "[To] Where are we going to go tomorrow?" or more naturally "Where are we going to go [to] tomorrow?" so we use "¿A dónde / Adónde vamos a ir mañana?"

But we can't say something like "[To] Where is the bathroom?" so that is just "¿Dónde está el baño?"

Unlike English the Spanish present progressive is very rarely, if ever, used to refer to future events.

The "a" is part of the informal future structure:

Ir (conjugated to the subject) + a + infinitive.

So in this case:

Vamos + a + ir.

Yes, it is common in Spanish with statements in future tense, because 'will' is equals to 'going to', totally and exactly. In Spanish I can say: 'A dónde vamos a ir mañana' o 'A dónde iremos mañana' (first one is more common even though second is the formal form). Those are the two ways that exist, there are no more. Source: Me, 'cause I' m native Spanish speaker.

"Where will we go tomorrow?" should work too I think.

"going to go" isn't redundant; in this case, "going to" is auxiliary phrasing that creates the tense/aspect. If you read "going to" as the informal "gonna", then it makes a lot more sense ("where we gonna go tomorrow?")

It appears this module only allows will/be going to in English and ir a/conjugated future tense in Spanish. And not even these are accepted in all four translation combinations consistently, the acceptance level of the Spanish conjugated futuro being the spottiest. My "iremos" worked here on 28 March 2014.

Because it is the present tense, not the future. "Where are we going (tomorrow). Tomorrow implies the future, but the sentence is in the present tense. Both English and Spanish use "going to" plus the base verb form as one way to create the future tense. The other way both languages do it is different. English uses "will" or "shall." Spanish uses a future verb ending conjugation.

I wonder, too, I had thought it was acceptable (and even common) to use the present tense if something else in the sentence denoted the future occurance.