Below is for BigQuery Standard SQL

#standardSQL
SELECT 
  ARRAY(
    SELECT AS STRUCT item, unit_cost, quantity
    FROM UNNEST(costs.item) item WITH OFFSET
    LEFT JOIN UNNEST(costs.unit_cost) unit_cost WITH OFFSET USING(OFFSET)
    LEFT JOIN UNNEST(costs.quantity) quantity WITH OFFSET USING(OFFSET)
  ) AS costs
FROM `project.dataset.table`   

if to apply to sample data from your question - result is (in JSON View)

[
  {
    "costs": [
      {
        "item": "cheese",
        "unit_cost": "2",
        "quantity": "1"
      },
      {
        "item": "ham",
        "unit_cost": "5",
        "quantity": "2"
      },
      {
        "item": "salad",
        "unit_cost": "8",
        "quantity": "1"
      }
    ]
  }
]

Try below

SELECT Session, Identifier, COUNT(1) AS dups
FROM `events.myevents`, UNNEST(Information)
GROUP BY Session, Identifier
HAVING dups > 1
ORDER BY Session

Should give you what you expect plus number of dups.
Like below (example)

Session Identifier                              dups     
abc123  e8d971a4-ef33-4ea1-8627-f1213e4c67dc    2    
abc345  1c62813f-7ec4-4968-b18b-d1eb8f4d9d26    3    

Below is for BigQuery Standard SQL

Based on limited info in your question - I guess you are missing STRING_AGG in the second query you presented in your question

It should be

SELECT
  CAST(property_hs_object_id.value AS String) AS deal_ID,
  (SELECT STRING_AGG(associations.associatedvids) FROM UNNEST(associations.associatedvids)) AS associated_contacts_ID
FROM hubspot_data.deals   

Update: answer on updated question

#standardSQL
SELECT 
  deal_ID,
  ARRAY(
    SELECT AS STRUCT 
      company_ID, 
      ( SELECT STRING_AGG(CAST(id AS STRING), ', ') 
        FROM t.contact_ID id
      ) AS contact_ID 
    FROM d.associations t
  ) AS contacts
FROM deals d