Consecutive transactions - firebase

Question

I was trying to simulate a situation where two users (on seperate devices) both run a Transaction at the same time. To imitate this, I made a List<String> of strings which would be added to the database without a delay between them.
However, only the first item in the List was added to the database, the second never arrived. What am I doing wrong? I am trying to have both items added to the database.
The call to the Transaction happens in the code below, along with the creation of the list:
List<String> items = new List<String>();
items.add("A test String 1");
items.add("A test String 2");
for (String q in questions)
{
database.updateDoc( q );
}
The code I use for updating the data in my database:
void updateDoc( String item ) async
{
var data = new Map<String, dynamic>();
data['item'] = item;
Firestore.instance.runTransaction((Transaction transaction) async {
/// Generate a unique ID
String uniqueID = await _generateUniqueQuestionCode();
/// Update existing list
DocumentReference docRef = Firestore.instance
.collection("questions")
.document("questionList");
List<String> questions;
await transaction.get(docRef)
.then (
(document) {
/// Convert List<dynamic> to List<String>
List<dynamic> existing = document.data['questions'];
questions = existing.cast<String>().toList();
}
);
if ( ! questions.contains(uniqueID) )
{
questions.add( uniqueID );
var newData = new Map<String, dynamic>();
newData['questions'] = questions;
transaction.set(docRef, newData );
}
/// Save question
docRef = Firestore.instance
.collection("questions")
.document( uniqueID );
transaction.set(docRef, data);
});
}
In reality, I have a few fields in the document I'm saving but they would only complicate the code.
I keep track of a list of documents because I need to be able to retreive a random document from the database.
When executing the first code snippet, only the first item in the list will be added to the database and to the list that keeps track of the documents.
No error is thrown in the debug screen.
What am I missing here?

Answer 1

As explained by Doug Stevenson in the comments under my question:
That's not a typical use case for a single app instance. If you're
trying to find out if transactions work, be assured that they do.
They're meant to defend against cases where multiple apps or processes
are making changes to a document, not a single app instance.
And also:
The way the Firestore SDK works is that it keeps a single connection
open and pipelines each request through that connection. When there
are multiple clients, you have multiple connection, and each request
can hit the service at a different time. I'd suspect that what you're
trying to simulate isn't really close to the real thing.