Can't create packages in Delphi 10.1 Berlin

Hi,

Environment: Windows 7 SP1
Installed: Delphi 10 Seattle and Delphi 10.1 Berlin

I can't build any package in Delphi 10.1 Berlin.
I have no problems when building the same packages in Seattle.
Even the simplest package with only 1 basic unit in the contains-section gives errors:
E2200 Package 'Package1' already contains unit 'Unit1'

package Package1;

{$R *.res}
{$IFDEF IMPLICITBUILDING This IFDEF should not be used by users}
{$ALIGN 8}
{$ASSERTIONS ON}
{$BOOLEVAL OFF}
{$DEBUGINFO OFF}
{$EXTENDEDSYNTAX ON}
{$IMPORTEDDATA ON}
{$IOCHECKS ON}
{$LOCALSYMBOLS ON}
{$LONGSTRINGS ON}
{$OPENSTRINGS ON}
{$OPTIMIZATION OFF}
{$OVERFLOWCHECKS OFF}
{$RANGECHECKS OFF}
{$REFERENCEINFO ON}
{$SAFEDIVIDE OFF}
{$STACKFRAMES ON}
{$TYPEDADDRESS OFF}
{$VARSTRINGCHECKS ON}
{$WRITEABLECONST OFF}
{$MINENUMSIZE 1}
{$IMAGEBASE $400000}
{$DEFINE DEBUG}
{$ENDIF IMPLICITBUILDING}
{$IMPLICITBUILD ON}

contains
  Unit1 in 'Unit1.pas';

end.

unit Unit1;
interface
const
  ourTest1 = 'Test1';
  ourTest2 = 'Test2';
implementation

end.

Any idea what's wrong ?
Regards

Freddy