Because of multicast communication several devices at the same time can send a LL acknowledgement as reaction to the same telegram.
Because of collision avoidance only one LL acknowledgement will reach the sender.
The LL acknowledgement priority is defined as follows: LL_BUSY > LL_NAK > LL_ACK
Example: suppose that 10 devices are addressed via the same multicast telegram, where 9 of them react with LL_ACK and one with LL_NAK, because of collision avoidance only the LL_NAK will reach the sender, which will make the sender repeat the telegram.
Multiple LL acknowledgements
Still according to the same example: the devices that already reacted with an LL_ACK to the (first) multicast telegram, will not process the repeated telegram. The original telegram can be distinguished from its repetitions by means of the repeat flag in the control field.