It turned out there had been two Marines with the same name and similar numbers in the camp. (Para. 14)
Meaning: It was found in the end that there had been two Marines with the same name and similar numbers in the camp.
turn out (in the above context): happen to be, be found in the end
"Turn out" has other meanings. Please read the following examples:
Turn the light out (= Turn the light off) before you leave. 你離開前, 請把燈關了。
Huge crowds turned out to see the foreign visitors. 一大群人蜂擁而出, 去看外賓。
This factory can turn out 100 cars a day. 這家工廠每天生產100輛汽車。