UPDATE: This article has now been updated to reflect a more detailed position of the nationality laws of the respective EU countries. The updated version can be found here
In light of Brexit, a considerable number of EU nationals are pushing through applications for recognition of their right to permanent residency in the UK in accordance with the EEA Regulations. Many are progressing to naturalise as British nationals especially those who have made the UK their home for a considerable number of years.
Many questions have arisen lately over which EU countries permit for Dual nationality which has prompted this posting.
In general terms those permitting dual nationality are : Belgium, Bulgaria, Croatia, Republic of Cyprus, Czech Republic, Denmark, Finland, France, Germany ( but only with other EU countries ) Greece, Hungary, Ireland, Italy, Latvia, Luxembourg, Malta, Poland, Portugal, Romania, Slovenia, Spain, Sweden & Switzerland.
Those countries who do not : Austria, Estonia, Lithuania, Netherlands, Norway & Slovakia.
There are ofcourse exceptions to the general practice and laws of the various countries and clarification should always be sought from one's Embassy or Consulate before embarking on the procedure to naturalise as a UK Citizen.
Source : European Union Democracy Observatory on Citizenship http://eudo-citizenship.eu/country-profiles