I likely will need to upload the actual Embarcadero 10.3 Studio code for this one. 

'Query' is a Tiboquery cut from Delphi 7 code compiled under 10.3.  'Query2' is newly inserted using ONLY 10.3.  'Query' will prepare through 'DB' and work properly under 10.3.   'Query2' will not prepare under 10.3. Other than the 'prepare' issue visually through the 10.3 Object Inspector, I have discerned no differences.

The DDL for the database is in a comment section of the code below.  Clicking the 'Create Data' button will put 4 rows into the table 'Stuff', but that is not needed to obtain the failure of the TiboQuery to prepare.  I am using ibo5_10_1_b2808_Source and Embarcadero 10.3 Studio.

[code]
unit Delete;

interface

uses
  Winapi.Windows, Winapi.Messages, System.SysUtils, System.Variants, System.Classes, Vcl.Graphics,
  Vcl.Controls, Vcl.Forms, Vcl.Dialogs, IB_Components, IB_Access, IBODataset,
  Data.DB, Vcl.StdCtrls;

type
  TForm1 = class(TForm)
    Button1: TButton;
    Query2: TIBOQuery;
    DB: TIBODatabase;
    Button2: TButton;
    Query: TIBOQuery;
    QueryNUMBER: TIntegerField;
    QueryENTERED: TDateTimeField;

    procedure Button1Click(Sender: TObject);
    procedure Button2Click(Sender: TObject);

  private
    { Private declarations }
  public
    { Public declarations }
  end;

var
  Form1: TForm1;

implementation

{$R *.dfm}
{   DDL for table 'Stuff'

CREATE TABLE Stuff

(
  NUMBER          INTEGER           NOT NULL,
  ENTERED         TIMESTAMP         NOT NULL
);
}

procedure TForm1.Button2Click(Sender: TObject);
var Idx: integer;
    TS: TDateTime;
begin
  Query.Filtered := false;
  Query.Open;
  for Idx := 1 to 3 do
    begin
      Query.Append;
      QueryNUMBER.Value := Idx;
      QueryENTERED.Value := Now;
      Query.Post;
    end;
//  Duplicate record
  TS := QueryENTERED.Value;
  Query.Append;
  QueryNUMBER.Value := 3;
  QueryENTERED.Value := TS;
  Query.Post;
  ShowMessage('Added 4 rows to table.');
end;

procedure TForm1.Button1Click(Sender: TObject);
var Last_Number: integer;
begin
  Query.First;
  Last_Number := 0;
  while not Query.Eof do
    begin
      if QueryNUMBER.Value = Last_Number then
        begin
          ShowMessage('Deleting Duplicate #' + QueryNUMBER.AsString);
          Query.Delete;
          continue;
        end;
      ShowMessage('Deleting # ' + QueryNumber.AsString);
      Query.Delete;
    end;

end;

end.

[/code]