Magnar cast a strategic damage spell on my army led by Lady Irane.  I was notified that the shieldman (henchman) had escaped the battle to my nearest town, and would be immobilized for 3 turns.  If I looked at the troops stationed in the town, the shieldman was there.  The army was not transported, and if I looked at it, the shieldman was there also (although greyed out in both cases).

After 3 turns, I was notified that the shieldman was available again at the town. Thinking that I needed to move him out of the town and travel to rejoin the army, I moved him out of the town.  To my surprise, the whole army appeared outside the town, transported from where they were.  Not only that, they appeared to be duplicated.  I had an army management screen showing the same army on both sides:

and on the map it appeared that I had two identical armies stacked:

When I saved and reloaded the game, the duplicate army disappeared.