collab: Avoid creating duplicate Stripe customers (#29566)

This PR makes it so we check for an existing Stripe customer by email
address before attempting to create a new one.

This should avoid the case where we end up creating multiple Stripe
customers for the same user.

Release Notes:

- N/A

crates/collab/src/api/billing.rs

@@ -321,6 +321,24 @@ async fn create_billing_subscription(
     let customer_id = if let Some(existing_customer) = &existing_billing_customer {
         CustomerId::from_str(&existing_customer.stripe_customer_id)
             .context("failed to parse customer ID")?
+    } else {
+        let existing_customer = if let Some(email) = user.email_address.as_deref() {
+            let customers = Customer::list(
+                &stripe_client,
+                &stripe::ListCustomers {
+                    email: Some(email),
+                    ..Default::default()
+                },
+            )
+            .await?;
+
+            customers.data.first().cloned()
+        } else {
+            None
+        };
+
+        if let Some(existing_customer) = existing_customer {
+            existing_customer.id
         } else {
             let customer = Customer::create(
                 &stripe_client,
@@ -332,6 +350,7 @@ async fn create_billing_subscription(
             .await?;
 
             customer.id
+        }
     };
 
     let success_url = format!(