The tax cannot be reduced according to § 27 Inheritance and Gift Tax Act (IGTA) if the first acquisition was not subject to the IGTA. A foreign tax – even if it is comparable to the German Inheritance and Gift Tax – does not come under § 27 para. 1 IGTA.
§ 27 IGTA is not contrary to the free movement of capital (Art. 63 Treaty on the Functioning of the European Union (TFEU), see also Decision of the European Court of Justice (C-123/15) – June 30, 2016).
§ 27 IGTA relieves multiple acquisitions of the same assets within ten years. For that matter, a tax relief of up to 50% is provided, depending on the time interval between the acquisitions.
For more details see Koeniger, The German Inheritance and Gift Tax, 2017, p. 294 et seqq.