for(i=0; i<arrLength; i++)
if(arr[i] == arr[i+1])

This is wrong.

wow, great idea!!

A function can't return multiple values. You don't need a break in this case because once "Eight" is returned, the function doesn't continue.

You have to pay attention: without break; the switch executes all code after finding a match (for example: if imput is 8 your function will return "Eight" and then "Nine" because a break; is missing)!

You don't have a case for 0. When number=0, your code needs to return "Zero". But because there is no 0 case, it goes to the default and returns number instead.

1. You need to return the final output and not print (using printf) to the console
2. Get rid of whitespace (the newline) characters
3. The case is only regarding strings but the input is an integer

Use markdown to display code using three backticks. See https://github.com/adam-p/markdown-here/wiki/Markdown-Cheatsheet#code-and-syntax-highlighting