He was not the only one, the Prussian Ordens-Liste of 1810 gives for 1805 (before October 15), with their 1810 titles:
63. King of Naples (Murat)
64. Duc de Benevent (Talleyrand)
65. Duc de Friaul (Duroc)
66. Duc de Neufchatel (Berthier)
67. Crown Prince of Sweden (Bernadotte)
68. Duc de Parma (Cambacérès)
The background is explained here:
The Prussian king had received 7 Grand'Aigles de la Légion d'Honneur for himself and members of his court (Friedrich Wilhelm let the French embassador Laforest select these other six). In exchange, he gave 7 of his highest decoration to the French dignitaries listed above.
Benadotte at that time was commanding the French troops in Hanover.