This paper deals with multi-packet detection for SC modulations (Single-Carrier) with FDE (Frequency Domain Equalization) to be used in the uplink of wireless systems. We consider iterative frequency-domain receivers that jointly perform the equalization, multipacket separation and channel decoding operations. Our receivers allow high throughput provided that the number of mobile terminals that is trying to access a given slot is equal to or below the multi-packet detection capacity Qmax. We design a MAC protocol able to handle a total number of mobile terminals above Qmax by controlling the number of contending mobile terminals. The paper evaluates the throughput of a saturated system with two retransmission policies: constant probability and exponential backoff. It proposes an optimal configuration for the initial packet transmission probability p associated to each mobile terminal for both policies. Our analytical results are validated using physical and MAC layer simulations, and show that a constant p outperforms the exponential backoff approach for the optimal p value.