It has been shown that Deca Iron ultra-triathletes (i.e. performing one Ironman triathlon per day for 10 days) progressively slowed down whereas daily performance in a Triple Deca Iron ultra-triathlon (i.e. performing 30 Ironman triathlons within 30 days) remained unchanged. We investigated pacing in the first Double Deca Iron ultra-triathlon (i.e. 20 Ironman triathlons within 20 days) held in history and hypothesized that athletes in Double Deca Iron ultra-triathlon would pace evenly (i.e. no decrease in daily performance over time) while athletes in Deca Iron ultra-triathlon would pace positively (i.e. become slower over time). Day 1 was the fastest in both the Deca and the Double Deca Iron ultra-triathlon. In the Deca Iron ultra-triathlon, Day 7 was the slowest and Day 5 was the slowest in the Double Deca Iron ultra-triathlon. In the Deca Iron ultra-triathlon, swimming and cycling splits were faster compared to the Double Deca Iron ultra-triathlon whereas in the Double Deca Iron ultra-triathlon, the running splits were faster than in the Deca Iron ultra-triathlon. In summary, in both the Deca and the Double Deca Iron ultra-triathlon, athletes paced evenly. In contrast to earlier findings in Deca and Triple Deca Iron ultra-triathlon, the last day was not the slowest day. This was most probably due to environmental conditions. The findings of this study may be important for exercise physiologists, coaches, athletes and enthusiasts of this sport.