Given an array of integers, return the first repeating element.
arr = [3, 6, 7, 1, 4, 6, 7]
The first repeating element in the array is 6.
Complete the following task based on the instruction and optional input provided.### Instruction:
Given an array of integers, return the first repeating element.

### Input:
arr = [3, 6, 7, 1, 4, 6, 7]

### Output:
The first repeating element in the array is 6.