How to return index position of first re-occurring number?

I am a new coder and new to C language. I want to know how to return the index position of a re-occurring number in an array. If the array was

values[10] = { 1 2 3 4 5 5 5 5 6 7} I would like the function to return the index position 4 as the re-occurrence of 5 begins from index position 4.

This is what I've managed to produce but don't know how to progress

int ReturningIndex(int values[], int numValues)
{
for(i=0;i<numValues;i++){
if(values[i + 1] == values[i] && values[i + 2] == values[i] && values[i + 3] 
== values[i]){
//This is where I'm confused....