There are countless unions between spanish and austrian Habsburgs, as well as spanish and french Bourbons+Orléans.
Also among french Borbons, and don't forget that Louis XIV imposed his illegitimate issue to all the princes du sang from the Orléans to the Bourbon-Condé and Bourbon-Conti, all ancestors of the present Orléans. Liechtenstein :
Pr. Alfred married Pss Henriette. They were the grand parents of the late Pr. Franz-Joseph II. Prussia :
Three of Friederich II the Great's sisters married the margraves of Brandenburg-Bayreuth, Brandenburg-Ansbach and Brandenburg-Schwedt.
Their brother Ferdinand married his niece Luise of Brandenburg-Schwedt.
I thought of going through a list of houses, and the last example in each of a marriage involving agnates. The unions did not all necessarily involve first cousins; they were comparable to marriages among common folk between persons with the same surnames (e.g. Franklin and Eleanor Roosevelt). Anyhow, please correct me on the following, and fill me in on additional information --
Great Britain: The Princess Mary (daughter of King George III) married Prince William of Gloucester and Edinburgh, upon which she became the Duchess of Gloucester and Edinburgh
Norway: none since the country acquired full independence in 1905
Denmark: Prince Knud (uncle of Queen Margrethe II) married in 1933 Caroline-Mathilde, a born princess of Denmark. However, a case can be made that a later example of a marriage between agnates in the house of Glücksburg would be the 1964 union of King Constantine II of the Hellenes, Prince of Denmark, and Princess Anne-Marie. The latter became not only a queen of Greece but also, a twice-princess of Denmark
Sweden: none to the best of my knowledge in the house of Bernadotte. Can anybody name an example? How about earlier dynasties (e.g. the house of Vasa)?
The Netherlands: none in the house of Orange (to the best of my knowledge). However, in the house of Nassau, Princess Caroline of Orange-Nassau married, in 1760, Prince Karl-Christian of Nassau-Weilburg. Is there a later example of a marriage involving agnates in the house of Nassau?
Belgium: none within the Belgian Coburgs. However, in the extended house, the 1875 union of Princess Louise (eldest daughter of King Leopold II) and Prince Ferdinand Philipp of Saxe-Coburg and Gotha (Kohary) would count as a marriage between agnates
Spain: the 1846 marriage of Queen Isabel II and Francisco de Asís María Fernando de Borbón, who became king-consort. Was there a later example of a union between two Spanish Bourbons? How about earlier history, involving the Spanish Habsburgs?
Portugal: the 1760 marriage of the future Queen Maria II and King Pedro III, who became joint sovereigns, was a union of two Braganzas. Is this correct, or was there a later example? How about earlier Portuguese dynasties>?
France (royal): Prince Henri, Count of Paris (born 1908) was the son of two members of the house of Orléans, each of whom was likewise a child of two members of the same house. His own marriage in 1931 was to Princess Isabelle of Orléans-Braganza. So this example would count as the latest example -- correct?
France (imperial): can anybody fill me in on the house of Bonaparte?
Bourbon-Two Sicilies: the 1868 marriage of Prince Alfonso, Count of Caserta and Princess Maria Antonietta
Bourbon-Parma: can anybody fill me in on this? Because the Bourbons are such a large royal house with multiple branches, I decided to restrict it to the Spanish Bourbons, the Sicilian Bourbons, and the Parmesan Bourbons. So I'm not sure if the marriage of Duke Roberto of Parma and Princess Maria Annuziata of Two Sicilies should count
Savoy (Sardinia, later Italy): can anybody fill me in on this?
Russia: the 1894 marriage of the Grand Duchess Xenia Alexandrovna and Grand Duke Alexander Mikhailovich. Was there a later example of a union involving Romanov agnates? How about earlier cases?
Greece: none in either the house of Wittelsbach or the specific Greek branch within the house of Glücksburg (the 1964 marriage of King Constantine II should be thought of as a union of Danish royal agnates)
Bavaria: the 1950 marriage of Prince Ludwig and Princess Irmengarde. Earlier examples of unions involving agnates within the house of Wittelsbach would be the 1900 first marriage of her father, Prince Rupprecht and Duchess Marie Gabrielle in Bavaria. Her paternal grandparents were Duke Maximilian in Bavaria and Princess Ludovika of Bavaria. Countess Palatine Maria Anna of Zweibrücken-Birkenfeld-Bischweiler (sister of King Maximilian I of Bavaria) married in 1780 Duke Wilhelm in Bavaria.
Prussia: can anybody fill me in on this?
Austria: the 1890 marriage of Archduchess Marie Valerie (daughter of Emperor Franz Joseph) and Archduke Franz Salvator of Austria-Tuscany. I don't believe there were any in-house unions involving the imperial line per se, so I decided to extend the discussion to the large house of Habsburg. Earlier examples would be the two marriages of Archduchess Elisabeth Franziksa of Austria-Hungary, a born Habsburg whose two husbands were both Habsburgs. Her first was a second cousin, Archduke Ferdinand Karl Viktor of Austria-Este; her second was a first cousin, Archduke Karl Ferdinand of Austria-Teschen. One of the four wives of Holy Roman Emperor Francis II (who became Emperor Franz I of Austria) was a fellow Habsburg, an archduchess of Austria-Este
Saxony (royal): none to the best of my knowledge; however, King Georg and Infanta Maria Ana of Portugal might count, if one is talking of the extended Saxon dynasty
Hanover: none since the separation of the British and German crowns in 1837 (to the best of my knowledge)
Württemberg: the 1845 marriage of Princess Catherine (daughter of King Wilhelm I) and Prince Friedrich
Is all this true? Aside from the above information I've asked to fill in, can anybody list cases in smaller dynasties -- such as the duchies, grand duchies, and principalities of Germany?