Thats partially correct, Duplicate detetction is also done in UM mode, when during re-ordering RLC checks for duplicacy.
As you know RLC operates in 3 modes, TM, UM and AM.
1. As the name suggest TM is quite transparent and provides no overhead, So RLC can use this mode in certain areas like broadcasting SI, Paging, and RRC message, where the operation needs to be uni-directional. TM is not used in user plane data.
2. UM modes also provides duplicate detection, reordering etc. UM is also uni-rectional where UM entity checks for duplicate using SN. To check for duplicacy the receving entity maintains a timer.
3. AM mode is bi-directional , as name suggest in Acknowledge mode, it waits for some sort of Ack-Nack to work on to avoid any further re-transmission of packets and causes delay. So AM can be used in certain places like Web browsing, file downloading etc where ACK-NACK is needed.