Two commanders of the same name, James H. Duncan served as Captain and Lt. Col., respectively of the 16th & 19th Mississippi.

Believe it was Lt. Col. of the 19th who served as the commander at Fort Gregg.

Richmond-Petersburg Campaign Site

Ninth Offensive Order of Battle:

16th Mississippi Infantry

Commander: Lieutenant Colonel James H. Duncan (April 2, 1865) regiment under Capt. J. H. Duncan, both under the immediate command of Lt. Col. James H. Duncan, of the 19th [Mississippi] regiment, in Fort Gregg. The 19th [Mississippi] and 48th [Mississippi] regiment were placed in Whitworth, and were under the personal command of Gen. Harris. Cols. R. H. Phipps and Joseph M. Jayne commanded the latter two regiments.”]

http://www.beyondthecrater.com/resources/units/conf-u/conf-inf/ms-inf/16th-mississippi-infantry/