Who is Duncan Idaho from God Emperor of Dune and what is their importance?

The original Duncan Idaho is House Atreides' black-haired, stony sharp featured Swordmaster, a skilled warrior, and trainer of Paul-Muad'Dib Atreides. He dies defending Paul and his mother Jessica against a Harkonnen/Sardauker invasion of Dune (Arrakis). Before joining up with the Atreides, Idaho is a Harkonnen slave on Giedi Prime, so his hatred of that Great House runs deep. Paul and his son Leto II, for 3,000 years the God Emperor, so enjoy Idaho's company and expertise, that they replace him whenever necessary by a fresh "ghola" grown from Tleilaxu axlotl tanks. As the novel opens, the current Idaho has served Leto for almost 60 years, but has been perverted and armed with an Ixian lasgun to assassinate Leto. He becomes the 19th Idaho to die an unnatural death as an enraged Worm rolls over him.