The only historical inacurracy?
There is very little in "The Duellists" that can be considered historically inaccurate. The uniforms and equipment are especially well researched and rendered. However, after doing a bit of study, I have come across what may be the only real historical inaccuracy in the entire film.
In Conrad's "The Duel" D'Hubert is mentioned as belonging to the 4th Hussars (a blue and red uniform) and Feraud to the 7th Hussars (a green and red uniform with gold trim). However, in Scott's "The Duellists" the filmmakers outfit D'Hubert in the uniform of the 3rd Hussars (a gray uniform with silver trim). This is of no real consequence except when one considers that D'Hubert and Feraud could not have historically had their encounter in the Russian snow in either the short story or the film.
After the horseback duel in Lubeck in 1806 the narrator states:
"Feraud was posted to Spain. D’Hubert remained in northern Europe. Six years later the Emperor’s Grand Army regrouped for Armageddon."
Being in the 7th Hussars, Feraud never would have gone to Spain. It is the 7th Hussars that in fact remained in northern Europe. D'Hubert would have gone to Spain with the 3rd (film) or 4th (short story) Hussars. Neither the 3rd or 4th ever served in Russia so the encounter in the snow would not have happened. They could, however, have conceivably run into one another around the time of Leipzig (The Battle of the Nations) in Germany in 1813, but as the story and film makes clear, "the duels of nations take absolute precedence."
The fault for this historical discrepancy lies more with Joseph Conrad than with the filmmakers. Conrad clearly places D'Hubert in Russia when in fact the 4th Hussars never served there.
For those interested, the post-1800 battle records of the three units are as follows:
3rd Hussars (D'Hubert's regiment in the film):
1805: Elchingen and Ulm (Note that in the film Laura asks D'Hubert, "Why weren't you killed at Elchingen?" D'Hubert responds "it was a hard day for the regiment.")
1806: Jena, Magdebourg, and Gollup
1807: Bartenstein, Langenheim, Hoff, and Guttstadt
1808: Tudela (Spain)
1809: Calcabellos, Tanoris, Banos, Tamanies and Alba de Tormes (Spain)
1810: Ciudad-Rodrigo, Almeida, Leria and Alcoluto (Spain)
1811: Redhina and Fuentes-de-Onoro (Spain)
1812: Arapiles (Spain)
1813: Vittoria (Spain)
1813: Leipzig
1814: Brienne, Montereau, and Sevenans
1815: Belfort
4th Hussars (D'Hubert's regiment in Conrad's short story):
1800: Neubourg, Ampfingen and Hohenlinden
1805: Austerlitz
1806: Jena and Lubeck
1807: Liebstadt and Mohrungen
1809: Alcanitz and Belchite (Spain)
1811: Stella, Chiclana, and Sagonte (Spain)
1813: Yecla and Col d'Ordal (Spain)
1813: Gross Beeren and Leipzig
1814: Lons-le-Saulnier, Saint Georges, and Lyon
1815: Ligny and Waterloo
7th Hussars (Feraud's regiment in both the film and short story):
1800: Engen, Nesselwangen, Feldkirch, and Salzbourg
1805: Mariazell, Affleng, and Austerlitz
1806: Gera, Zehbenick, Prentzlow, Stettin, Lubeck, Czenstowo, and Golymin
1807: Eylau, Heilsberg, and Konisberg
1809: Peising, Ratisbonne, Raab, Wagram and Znaim
1812: Vilna, Smolensk, Ostrowro, and Borodino (Russian Campaign)
1813: Borna, Altenbourg, Leipzig, and Hanau
1814: Vauchamps, Montereau, Reims, Laon, and Paris
1815: Fleurus and Waterloo