Two battles for the price of one??

[LDiCesare]

Besides, there was no random event in the province.

How can you know there's no random event in an indy province?
Depending on how (14) is implemented, I wouldn't rule out my hypothesis. Unfortunately it's almost impossible to test it.

[archaeolept]

there could have been a random event. However, random events occur before movement, so your explanation cannot work.

[The DarkOne]

I'm pretty sure that the additional battle wasn't because of a random event. All extra battles caused by random events, barbarian raids etc show up as an independent army that attacks you but in this case it was my army that was the attacking one in both battles.