How do you handle duplicate messages in a message queue?

i've been using message queues a lot recently, especially RabbitMQ, and i'm hitting that common problem of making sure messages are processed just once. RabbitMQ gives you at-least-once delivery, sure, but real exactly-once processing means you have to deal with duplicate messages yourself. so, what do you usually do to get rid of duplicates on the consumer end? are there any specific ways or tools that work really well for this? i'm wondering about the downsides, like how much it slows things down compared to being absolutely sure you aren't processing the same message twice