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. 你离开前, 请把灯关了。
Huge crowds turned out to see the foreign visitors. 一大群人蜂拥而出, 去看外宾。
This factory can turn out 100 cars a day. 这家工厂每天生产100辆汽车。