I have found the following record for my grandfather ALBERT DOUPLOY and have downloaded a copy of the relevant marriage certificate:

Name Albert Douploy
Event Type Marriage
Event Date 03 Nov 1917
Event Place Durban, Natal, South Africa
Gender Male
Age 24
Birth Year (Estimated) 1893
Spouse's Name Gladys Muriel Wheeler
Spouse's Gender Female
Spouse's Age 19
Spouse's Birth Year (Estimated) 1898
Reference ID 2203
GS Film number 1795576
Digital Folder Number 004236646
Citing this Record
"South Africa, Natal Province, Civil Marriages, 1845-1955," index and images, FamilySearch (https://familysearch.org/pal:/MM9.1.1/KDCR-5D1 : accessed 25 Oct 2014), Albert Douploy and Gladys Muriel Wheeler, 03 Nov 1917; citing Durban, Natal, South Africa; 02641; 1795576.

His occupation is given on the certificate as 'soldier', The witnesses to the marriage were Gladys's father Robert Wheeler and Florence Telfer. Gadys and Albert were divorced before 1932. Their son, my father, was known as Ronald DU PLOOY not DOUPLOY.

The only other reference I can find to the name DOUPLOY is the marriage of Maria Elizabeth Douploy who married a Charles William Smith in Durban on 12 Mar 1910. At this time the father of Maria was deceased and the mother was the legal guardian. Maria was born about 1890 and Charles was born 1883 and his occupation is given as Coach Painter.

LDS Microfilm #1795572
Natal Marriages
Item 1
Entry #257
Husband: Charles William SMITH
Wife: Maria Elizabeth DOUPLOY
Date: 3/12/1910

I am still looking for any information about my grandfather who I think died in Durban in the late 1950s or early 1960s. There are records of deaths for Albert du Plooy in the Pietermaritzburg Archives but I do not think that these are him. Since I live in Ireland the only way I can research is online. I anyone has any information about the surname DOUPLOY I would appreciate any help I can get.