Azure table example in F#. Why I need to implement members twice?

AzuteTableStorage.fsx

#r "nuget: Azure.Data.Tables"

open System
open Azure
open Azure.Data.Tables // Namespace for Table storage types
open System.Linq
open System.Collections.Generic

type Customer(firstName, lastName, email: string, phone: string) =
    let timestamp : Nullable<DateTimeOffset> = Nullable();
    let etag = ETag()
    interface ITableEntity with       
        member val ETag = etag with get, set
        member val PartitionKey = lastName with get, set
        member val RowKey = firstName with get, set
        member val Timestamp = timestamp with get, set
        
    new() = Customer(null, null, null, null)
    
    member val Email = email with get, set
    member val PhoneNumber = phone with get, set
    member val ETag = etag with get, set
    member val PartitionKey = lastName with get, set
    member val RowKey = firstName with get, set
    member val Timestamp = timestamp with get, set
 
// Error out  
//type Customer(firstName, lastName, email: string, phone: string) =
//    let timestamp : Nullable<DateTimeOffset> = Nullable();
//    let etag = ETag()
//    interface ITableEntity with       
//        member val ETag = etag with get, set
//        member val PartitionKey = lastName with get, set
//        member val RowKey = firstName with get, set
//        member val Timestamp = timestamp with get, set
//        
//    new() = Customer(null, null, null, null)
//    
//    member val Email = email with get, set
//    member val PhoneNumber = phone with get, set


let storageConnString =
    "DefaultEndpointsProtocol=http;AccountName=devstoreaccount1;AccountKey=Eby8vdM02xNOcqFlqUwJPLlmEtlCDXJ1OUzFT50uSRZ6IFsuFq2UVErCz4I6tq/K1SZFPTOtr/KBHBeksoGMGw==;TableEndpoint=http://127.0.0.1:1002/devstoreaccount1;"

let tableClient = TableServiceClient(storageConnString)

let table = tableClient.GetTableClient("people")

// Create the table if it doesn't exist.
table.CreateIfNotExists() |> ignore

let customer = Customer("Walter", "Harp", "Walter@contoso.com", "425-555-0101")
table.AddEntity(customer) |> ignore


let customer1 =
    Customer("Jeff", "Smith", "Jeff@contoso.com", "425-555-0102")

let customer2 =
    Customer("Ben", "Smith", "Ben@contoso.com", "425-555-0103")

let entityList = [ customer1; customer2]

// Create the batch.
let addEntitiesBatch = List<TableTransactionAction>()

// Add the entities to be added to the batch.
addEntitiesBatch.AddRange(entityList.Select(fun e -> TableTransactionAction(TableTransactionActionType.Add, e)))

// Submit the batch.
let response = table.SubmitTransactionAsync(addEntitiesBatch).ConfigureAwait(false)

let results = table.Query<Customer>("PartitionKey eq 'Smith'")

printfn "ALL customers"
for customerB in results do
    let customer = customerB :> ITableEntity    
    printfn $"customer: {customer.RowKey} {customer.PartitionKey}"


let rangeResult = table.Query<Customer>("PartitionKey eq 'Smith' and RowKey lt 'M'")

printfn "The m customers"
for customerB in rangeResult do
    let customer = customerB :> ITableEntity    
    printfn $"customer: {customer.RowKey} {customer.PartitionKey}"
let singleResult = table.GetEntityAsync<Customer>("Smith", "Ben").Result.Value 
let s = singleResult :> ITableEntity
printfn "A customer"
printfn $"customer: {s.RowKey} {s.PartitionKey}"

try
    singleResult.PhoneNumber <- "425-555-0103"
    table.UpdateEntity(singleResult, ETag("etag"), TableUpdateMode.Replace) |> ignore
    Console.WriteLine("Update succeeeded")
with e ->
    Console.WriteLine("Update failed")

try
    singleResult.PhoneNumber <- "425-555-0104"
    ignore(table.UpsertEntity(singleResult, TableUpdateMode.Replace))
    Console.WriteLine("Update succeeeded")
with e ->
    Console.WriteLine("Update failed")

let subsetResults = query{
    for customer in table.Query<Customer>() do 
    select customer.Email
}

printfn "Only email"
subsetResults |> Seq.iter (printfn "%s")