Bertie Bott's says to gain an attack token when playing an ally - simple enough.
Polyjuice Potion says to copy the effect of an ally - also simple enough.
Will playing Polyjuice Potion to copy an ally previously played cause Bertie Bott's ability to trigger (gaining a second attack token)?
The key is in the phrasing. Polyjuice potion copies the effect, but not the actually ally. It is basically an item that has the abilities of an ally.