When two pokemon with the same choice item attack each others they are both locked into the slower pokemon move, causing one of them to struggle.
For example, if a choice scarfed pokemon switches into another scarfed pokemon attack, the pokemon will be locked into that move.
Basically it makes the other scarf lock into the last used (scarfed) move.
Most of the times the two pokemons have 8 possible diffrent moves and that's what causes one of the two pokemon to struggle.
If the two pokemon share the same move the pokemon that gets hit last will also be locked into that move. (No struggle)