Creating a Relationship with duplicate values in power Query in Excel

Hi geemian93, with your's sample data it is simple (but you have probably different data)

Result

let
    TransactionTable = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WMtQ31DcyMDJW0lEyNAACI1MQy8jUAEiFBrsoxepEK5miqTEGSRoZICuxRFMC1m+IosTQAFUNRNYYYVMsAA==", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [DateDoc = _t, #"No." = _t, #"AmountTrans." = _t, Currency = _t]),
    ExchRateTable = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("bc7BDcAgCAXQXTibCghaBugCTXoy7r9Ga4K94IXLy//83oEyZUYukOB67u/iYYqIMNJEjkisjiUgVxVHicl2LtQdmmPd4D+oRRRdeAYUa2utxSQrORJueufT8QI=", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [#"Daterep." = _t, Currency = _t, #"CurrencyUSD/EUR Rate" = _t]),
    TransactionTableChangedType = Table.TransformColumnTypes(TransactionTable,{{"No.", Int64.Type}, {"AmountTrans.", type number}, {"DateDoc", type date}}, "sk-SK"),
    ExchRateTableChangedType = Table.TransformColumnTypes(ExchRateTable,{{"Daterep.", type date}, {"CurrencyUSD/EUR Rate", type number}}, "en-US"),
    MergedQueries = Table.NestedJoin(TransactionTableChangedType, {"DateDoc"}, ExchRateTableChangedType, {"Daterep."}, "ExchRateTableChangedType", JoinKind.LeftOuter),
    #"Expanded ExchRateTableChangedType" = Table.ExpandTableColumn(MergedQueries, "ExchRateTableChangedType", {"CurrencyUSD/EUR Rate"}, {"CurrencyUSD/EUR Rate"})
in
    #"Expanded ExchRateTableChangedType"

Make sample tables with multi currency transactions and rates together with resulting table. Now it works for single ccy pair. 

let
    combine = transactions & Table.RenameColumns(rates, {"Daterep.", "Date"}),
    sort = Table.Sort(combine,{{"Date", Order.Ascending}, {"Doc No.", Order.Ascending}}),
    rate_down = Table.FillDown(sort,{"USD/EUR Rate"}),
    filter = Table.SelectRows(rate_down, each ([Currency] = "USD")),
    amount = Table.AddColumn(filter, "EUR Amount", each [#"AmountTrans."] * [#"USD/EUR Rate"])
in
    amount