doppelganger exercise for the exam

subjects/doppelganger.en.md

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+## doppelganger
+
+## **WARNING! VERY IMPORTANT!**
+
+For this exercise a function will be tested **with the exam own main**. However the student **still needs** to submit a structured program:
+
+This means that:
+
+- The package needs to be named `package main`.
+- The submitted code needs one declared function main(```func main()```) even if empty.
+- The function main declared needs to **also pass** the `Restrictions Checker`(illegal functions tester). It is advised for the student to just empty the function main after its own testings are done.
+- Every other rules are obviously the same than for a `program`.
+
+### Instructions
+
+You are given 2 strings. Find out if first string contains second string. If it does, return index of the first string where second string occurs last time. If it does not contain, return "-1"
+
+### Expected function
+
+```go
+package main
+
+func DoppelGanger(big, little string) int {
+
+}
+```
+
+```go
+package main
+
+import (
+	"fmt"
+)
+
+func main() {
+	var result int
+
+	result = DoppelGanger("aaaaaaa", "a")
+	fmt.Println(result) // 6
+
+	result = DoppelGanger("qwerty", "t")
+	fmt.Println(result) // 4
+
+	result = DoppelGanger("a", "b")
+	fmt.Println(result) // -1
+}
+```