How to add data to Map copying existing values based on List of identifiers

Sorry for the poor title but it is rather hard to describe my use case in a short sentence.

Context

I have the following model:

typealias Identifier = String

data class Data(val identifier: Identifier,
                val data1: String,
                val data2: String)

And I have three main data structures in my use case:

• A Set of Identifiers that exist and are valid in a given context. Example:

val existentIdentifiers = setOf("A-1", "A-2", "B-1", "B-2", "C-1")

• A Map that contains a List of Data objects per Identifier. Example:

val dataPerIdentifier: Map<Identifier, List<Data>> = mapOf(
    "A-1" to listOf(Data("A-1", "Data-1-A", "Data-2-A"), Data("A-1", "Data-1-A", "Data-2-A")),
    "B-1" to listOf(Data("B-1", "Data-1-B", "Data-2-B")),
    "C-1" to listOf(Data("C-1", "Data-1-C", "Data-2-C"))
)

• A List of Lists that group together the Identifiers that should share the same List<Data> (each List includes always 2 Identifiers). Example

val identifiersWithSameData = listOf(listOf("A-1", "A-2"), listOf("B-1", "B-2"))

Problem / Use Case

The problem that I am trying to tackle stems from the fact that dataPerIdentifier might not contain all identifiersWithSameData given that existentIdentifiers contains such missing Identifiers. I need to add those missing Identifier to dataPerIdentifier, copying the List<Data> already in there.

Example

Given the data in the Context section:

A-1=[Data(identifier=A-1, data1=Data-1-A, data2=Data-2-A), 
     Data(identifier=A-1, data1=Data-1-A, data2=Data-2-A)], 
B-1=[Data(identifier=B-1, data1=Data-1-B, data2=Data-2-B)], 
C-1=[Data(identifier=C-1, data1=Data-1-C, data2=Data-2-C)]

The desired outcome is to update dataPerIdentifier so that it includes:

A-1=[Data(identifier=A-1, data1=Data-1-A, data2=Data-2-A), 
     Data(identifier=A-1, data1=Data-1-A, data2=Data-2-A)], 
B-1=[Data(identifier=B-1, data1=Data-1-B, data2=Data-2-B)], 
C-1=[Data(identifier=C-1, data1=Data-1-C, data2=Data-2-C)], 
A-2=[Data(identifier=A-2, data1=Data-1-A, data2=Data-2-A), 
     Data(identifier=A-2, data1=Data-1-A, data2=Data-2-A)]

The reason is that existentIdentifiers contains A-2 that is missing in the initial dataPerIdentifier Map. B-2 is also missing in the initial dataPerIdentifier Map but existentIdentifiers does not contain it, so it is ignored.

Possible solution

I have already a working code (handleDataForMultipleIdentifiers() method is the one doing the heavy lifting), but it does not feel to be the cleanest or easiest to read:

fun main(args: Array<String>) {
    val existentIdentifiers = setOf("A-1", "A-2", "B-1", "C-1")

    val dataPerIdentifier: Map<Identifier, List<Data>> = mapOf(
        "A-1" to listOf(Data("A-1", "Data-1-A", "Data-2-A"), Data("A-1", "Data-1-A", "Data-2-A")),
        "B-1" to listOf(Data("B-1", "Data-1-B", "Data-2-B")),
        "C-1" to listOf(Data("C-1", "Data-1-C", "Data-2-C"))
    )

    val identifiersWithSameData = listOf(listOf("A-1", "A-2"), listOf("B-1", "B-2"))

    print("Original Data")
    println(dataPerIdentifier)

    print("Target Data")
    println(dataPerIdentifier.handleDataForMultipleIdentifiers(identifiersWithSameData, existentIdentifiers))
}

fun Map<Identifier, List<Data>>.handleDataForMultipleIdentifiers(identifiersWithSameData: List<List<Identifier>>, existentIdentifiers: Set<Identifier>)
        : Map<Identifier, List<Data>> {
    val additionalDataPerIdentifier = identifiersWithSameData
        .mapNotNull { identifiersList ->
            val identifiersWithData = identifiersList.find { it in this.keys }
            identifiersWithData?.let { it to identifiersList.minus(it).filter { it in existentIdentifiers } }
        }.flatMap { (existentIdentifier, additionalIdentifiers) ->
            val existentIdentifierData = this[existentIdentifier].orEmpty()
            additionalIdentifiers.associateWith { identifier -> existentIdentifierData.map { it.copy(identifier = identifier) } }.entries
        }.associate { it.key to it.value }

    return this + additionalDataPerIdentifier
}

typealias Identifier = String

data class Data(val identifier: Identifier,
                val data1: String,
                val data2: String)

So my question is: how can I do this in a simpler way?